Attenuated uracil auxotroph of an apicomplexan and use thereof

ABSTRACT

Uracil auxotroph mutants of apicomplexans are provided which lack a functional carbamoyl phosphate synthase II (CPSII) enzyme. Also provided are  T. gondii  autoxtroph mutants which express exogenous antigens, and methods of protecting an animal against a  T. gondii  and non- T. gondii  disease.

INTRODUCTION

This patent application is the National Stage of International Application No. PCT/US2008/087081 filed Dec. 17, 2008, which claims the benefit of priority from U.S. Ser. No. 11/962,584 filed Dec. 21, 2008, teachings of each of which are herein incorporated by reference in their entirety.

This invention was supported in part by funds from the U.S. government (NIH Grant No. R01 AI41930) and the U.S. government may therefore have certain rights in the invention.

BACKGROUND OF THE INVENTION

Toxoplasma gondii is an obligate intracellular parasite capable of infecting most warm-blooded vertebrates and many nucleated cell types. Parasite transmission occurs orally through ingestion of tissue cysts or sporozoites from feline feces in contaminated soil, food, and water. Infection typically results in an asymptomatic primary infection that leads to a chronic latent infection affecting 30% of the world's population (Carruthers (2002) Acta Trop. 81:111-122). Following oral ingestion of tissue or oocyst cysts, parasites are released into the gut mucosa where they infect host cells and transform into the rapidly replicating tachyzoite stage. Rapidly replicating tachyzoites disseminate widely throughout the host reaching most organs and the brain. Host immune pressure is thought to trigger differentiation of tachyzoites into slow growing bradyzoites and development of tissue cysts. Despite the potent Th-1 acquired immunity that is elicited by primary infection, tissue cysts persist in immune privileged sites such as the brain for the life of the host. The reactivation of bradyzoites to tachyzoite differentiation in brain cysts leads to recrudescent and life threatening Toxoplasmic encephalitis in AIDS patients (Luft and Remington (1992) Clin. Infect. Dis. 15:211-222). T. gondii primary infections in pregnancy also lead to spontaneous abortion or severe CNS damage in neonates. As T. gondii is the 3^(rd) leading cause of food-born illness in the U.S., it is a significant human pathogen and therefore understanding the mechanisms underlying the development of protective immunity in response to infection is of high importance to development of vaccines.

T. gondii is now a widely recognized model for host response mechanisms. During active infection, T. gondii induces a potent systemic Th-1 inflammatory response that results in life long CD8⁺ T cell-mediated immune control of the infection. Infection triggers the innate response through a MyD88-dependent pathway resulting in IL-12-independent production of IFN-γ by NK and T cells leading to the recruitment of neutrophils and macrophages to the site of infection (Scanga, et al. (2002) J. Immunol. 168:5997-6001; Mun, et al. (2003) Int. Immunol. 15:1081-1087; Scharton-Kersten, at al. (1996) Exp. Parasitol. 84:102-114). Concomitant with the innate response, the development of the acquired Th-1 response is driven by secretion of IL-12 from neutrophils, macrophages and DCs that increases inflammatory cell infiltration, activates APCs and enhances production of IFN-γ by T cells and NK cells leading to the cell-mediated immune control (Bennouna, et al. (2003) J. Immunol. 171:6052-6058; Gazzinelli, et al. (1994) J. Immunol. 153:2533-2543).

Certain mechanisms of the immune response and key mediators of host immune control have been defined. Previous studies of host responses have typically used replicating and infectious strains of T. gondii that widely disseminate and cause extensive host tissue destruction and associated host-derived inflammatory responses. Other immune response models are based on studies using whole parasite antigen or parasite components (Scanga, et al. (2002) supra; Mun, et al. (2003) supra; Scharton-Kersten, et al. (1996) supra; Bennouna, et al. (2003) supra; Gazzinelli, et al. (1994) supra; Aliberti, et al. (2000) Nat. Immunol. 1:83-87). These host response and vaccine models reveal that immunization with weakened, but living and invasive T. gondii parasites results in complete protection against lethal challenge infections (Waldeland and Frenkel (1983) J. Parasitol. 69:60-65; Snzuki and Remington (1988) J. Immunol. 140:3943-3946; Bourguin, et al. (1998) Infect. Immun. 66:4867-4874; Kasper, et al. (1985) J. Immunol. 134:3426-3431). However, there is a need in the art for live attenuated vaccines to T. gondii and other apicomplexans. The present invention meets this need in the art.

SUMMARY OF THE INVENTION

The present invention features an attenuated uracil auxotroph mutant of Toxoplasma gondii containing a nucleic acid molecule encoding an exogenous protein. In certain embodiments, the mutant lacks a functional carbamoyl phosphate synthetase II enzyme.

In some embodiments, the exogenous protein is a non-Toxoplasma gondii antigen, such as a bacterial, viral, fungal, parasitic or tumor antigen, or the exogenous protein produces a non-Toxoplasma gondii antigen such as a lipid or polysaccharide. A vaccine for protection against infection by T. gondii and a non-T. gondii disease is also provided, as is the use of the mutants and vaccines of the invention to generate an immune response in an animal to Toxoplasma gondii and/or a non-T. gondii antigen.

In other embodiments the exogenous protein is a therapeutic antibody, protein, enzyme or peptide; or a human therapeutic target protein or enzyme. Use of mutant expressing human therapeutic target protein or enzyme in a live, whole-cell screening assay for identifying effectors of the human therapeutic target protein or enzyme is also provided.

The present invention also features the use of a mutant of the invention for sensing the host cell environment or specific molecules present or absent in the host cell environment. In particular embodiments, the molecule is a pyrimidine.

An addition feature of the present invention is an attenuated uracil auxotroph mutant of Toxoplasma gondii having a mutation at one or more amino acid residues selected from amino acid residue 171-229, 345, 348, 385, 435, 454-470, 533, 873-910, 1316, 1318, 1336, 1430, 1530, 1592-1628, and 1649 of SEQ ID NO:2, and a vaccine containing the same for generating an immune response in an animal to Toxoplasma gondii.

The present invention further pertains to an attenuated uracil auxotroph mutant of an apicomplexan, wherein said mutant lacks a functional CPSII enzyme comprising the amino acid sequence set forth in SEQ ID NO:8, and a vaccine containing the same. In one embodiment, the attenuated pyrimidine auxotroph mutant features the replacement of all or a portion of the coding sequence of the CPSII enzyme with a nucleic acid encoding a marker protein. In another embodiment, the apicomplexan is Toxoplasma gondii. In particular embodiment, the Toxoplasma gondii CPSII mutant is used in a method for generating an immune response in an animal to Toxoplasma gondii

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the genomic DNA and cDNA derived clones obtained from the CPSII locus of T. gondii. Genomic DNA clones and their names are shown in solid, uniformly-sized boxes, cDNA clones are shown in solid boxes, which alternate in size. The complete T. gondii CPSII cDNA is encoded on 37 exons spanning about 24 kb of the genomic DNA.

FIG. 2 shows the percent survival of mice immunized with cps1-1 knock-out via different routes of administration. FIG. 2A, C57Bl/6 mice were unimmunized or immunized with either 1× s.c., 2× s.c., 1× i.p. or 2× i.p. and challenged 1 month after final immunization with 10³ RH and percent survival was measured. FIGS. 2B and 2C, C57BL/6 mice were unimmunized or immunized once with either cps1-1 knock-out alone or DC, pMAC, or PEC loaded ex vivo for 12 hours with cps1-1 knock-out. Two months (FIG. 2B) or 6 months (FIG. 2C) post-i.v. immunization, mice were challenged with 10² tachyzoites of RH i.p. and percent survival was measured out to 30 days post-challenge. Data represents one experiment performed with 6 mice per immunization group. In FIGS. 2B and 2C, statistical significance was calculated using Kaplan-Meier product limit test.

FIG. 3 shows the effect of antibody depletion of T cells, lack of B cells, and adoptive transfer of immune cells on survival against lethal challenge. C57Bl/6 wild-type and μMT mice were immunized following an established protocol. One month after final immunization, wild-type mice were treated with either control Ig or antibody specific for CD4, CD8, or both CD4 and CD8 (FIG. 3A). FIG. 3B, both immunized and unimmunized μMT mice were left untreated, simultaneously challenged with 10³ RH parasites i.p. and percent survival was measured. FIG. 3C, C57Bl/6 mice were immunized as described above, then three weeks post-final immunization whole splenocytes, CD19⁺ and CD8⁺ splenocytes were harvested and either 4×10⁷ whole splenocytes, 1×10⁷ CD8⁺ T cells, or 5×10⁶ CD19⁺ B cells were transferred to naïve recipient mice. Twenty-four hours after transfer mice were challenged with 10³ RH parasites and monitored for survival. All experiments were performed with n=4 per group and repeated twice with similar results. The data are representative of the two experiments with similar results.

FIG. 4 shows peritoneal excudate inflammatory cell recruitment in response to infection with cps1-1 knock-out as compared to highly virulent strain RH. C57Bl/6 mice were infected i.p. with 1×10⁶ cps1-1 knock-out or 1×10³ RH parasites and total PECs were harvested at Days 0, 2, 4, 6, and 8. FIG. 4A, total PECs were analyzed by flow cytometry and total cell numbers recovered are presented. FIGS. 4B-4F respectively show numbers of granulocytes (GR-1⁺ CD68⁺ in R3), macrophages (CD68⁺), inflammatory macrophages (GR-1⁺ CD68⁺), and B lymphocytes (CD19⁺), and T lymphocytes (CD3⁺) upon cps1-1 vaccination and RH infection, whereas FIGS. 4F and 4G show CD3⁺ CD4⁺ and CD3⁺ CD8⁺ T lymphocyte numbers upon cps1-1 vaccination (FIG. 4F) or RH infection (FIG. 4G). The data are mean absolute numbers (upper panel) or percentages of total events recorded (lower panel) and are representative of two experiments that had similar outcomes. P values are based on unpaired two tailed Students T test.

FIG. 5 shows systemic Th-1 cytokine production in response to infection with cps1-1 or RH. C57Bl/6 mice were infected i.p. with 1×10⁶ cps1-1 or 1×10³ RH parasites and sera were taken at Days 0, 2, 4, 6, and 8. Serum levels of cps1-1- and RH-induced production of IFN-γ (FIG. 5A), IL-12p40 (FIG. 5B), and IL-12p70 (FIG. 5C) were measured by ELISA. The data presented are representative of three experiments with similar results and indicate the mean±SEM. P values are based on unpaired two tailed Students t-test and are as follows: FIG. 5A, p=*0.03, **0.001, ***0.0001, ****0.0001, ^(†)0.03, ^(††)0.0001; FIG. 5B, p=*0.0001, **0.04, ***0.0001, ****0.0001, ^(†)0.001; FIG. 5C, p=*0.001, **0.0001, ***0.0001, ^(†)0.04, ^(††)0.02.

FIG. 6 shows PEC and splenocyte Th-1 cytokine production in response to infection with cps1-1 or RH. C57Bl/6 mice were infected i.p. with 10⁶ cps1-1 or 10³ RH and whole PECs or splenocytes were harvested at Day 0, 2, 4, 6, and 8. PECs (FIGS. 6A-6C) were plated at 1×10⁶ cells/ml and splenocytes (FIGS. 6D-6F) were plated at 5×10⁶ cells/ml. All cells were cultured for 24 hours. Supernatants were then assayed for IFN-γ (FIGS. 6A and 6D), IL-12p40 FIGS. 6B and 6E), and Il-12p70 (FIGS. 6C and 6F) by ELISA. Day 0 controls represent control injection of PBS i.p. All experiments were performed with n=4 mice per group. The data presented are representative of two experiments with similar results and indicate the mean±SEM. P values are based on the unpaired two tailed Students t-test and are as follows: FIG. 6A, p=*0.005, ^(†)0.001, ^(††)0.04; FIG. 6B, p=*0.03, **0.0001, ^(†)0.0001, ^(††)0.04; FIG. 6C, p=*0.007, **0.012, ***0.001, ^(†)0.005; FIG. 6D, p=*0.001, **0.006, ***0.0001, ^(†)0.001; FIG. 6E, p=*0.02, **0.0001, ***0.002, ****0.02, ^(†)0.0001; FIG. 6F, no significant differences.

FIG. 7 shows a sequence alignment of CPSII genes from Escherichia coli (Ec; SEQ ID NO:26), Toxoplasma gondii (Tg; SEQ ID NO:2), Plasmodium falciparum (Pf; SEQ ID NO:27), Babesia bovis (Bb; SEQ ID NO:28), Trypanosoma cruzi (Tc; SEQ ID NO:29), Leishmania major (Lm; SEQ ID NO:30), Saccharomyces cerevisiae (Sc; SEQ ID NO:31), Homo sapiens (Hs; SEQ ID NO:32). Locations of indels in Toxoplasma gondii and locations where point mutations were constructed are underlined.

DETAILED DESCRIPTION OF THE INVENTION

T. gondii has a complete pathway for the de novo biosynthesis of pyrimidines (Schwartzmann and Pfefferkorn (1981) J. Parasitol. 67:150-158; Asai, et al. (1983) Mol. Biochem. Parasitol. 7:89-100). UMP, the first major end-product of the pathway, is synthesized from bicarbonate, glutamine, ATP, aspartate, and phosphoribosyl pyrophosphate (P-rib-PP) is catalyzed by six major enzymes: carbamoyl phosphate synthase (CPS), aspartate transcarbamylase (ATC), dihydroorotase (DHO), dihydroorotase dehydrogenase (DHOD), orotate phosphoribosyl transferase (OPT), and orotidylate decarboxylase (ODC or URA3). The pathway begins with CPS which combines glutamine, ATP and bicarbonate to form carbamoyl phosphate. The glutamine-specific CPS activity involved in de novo pyrimidine biosynthesis is referred to as CPSII and the enzyme is typically localized in the nucleolus of eukaryotic cells (Davis (1986) Microbiol. Reviews 50:280-313). ATC then combines carbamoyl phosphate and aspartate to form carbamoyl aspartate. The third reaction, catalyzed by DHO, yields dihydroorotate. DHOD then oxidizes dihydroorotate to orotate with the reduction of NAD. OPT then phosphoribosylates orotate to OMP. The sixth step, catalyzed by OMP Decarboxylase (URA3), converts OMP to UMP. UMP is the precursor of all pyrimidine nucleotides and deoxyribonucleotides.

In the Urea Cycle of ureotelic animals, carbamoyl phosphate is combined with ornithine, derived from ammonia, to form citrulline during de novo arginine biosynthesis. The CPS involved in arginine biosynthesis is referred to as CPSI. In some eukaryotes such as yeast, where CPSI is cytosolic, mutants of CPSII are a bit leaky because of some “mixing” of these two pools of carbamoyl phosphate. In many eukaryotes, CPSI is confined to the mitochondrial matrix and carbamoyl phosphate produced from CPSI in the Urea Cycle is unavailable to the carbamoyl phosphate “pool” which feeds into de novo pyrimidine biosynthesis (Davis (1986) supra). There is no mixing of CPSI and CPSII in T. gondii due to either sequestering of CPSI to a compartment such as the mitochondria or a lack of CPSI type activity in T. gondii. Thus, the CPSII involved in the de novo biosynthesis of pyrimidines is the first committed step of the de novo pathway of pyrimidine synthesis in T. gondii.

Comparative studies across many genera demonstrate extensive diversity in the de novo pathway's regulatory mechanisms, in the structure of its enzymes, and in the organization of the genes which encode the enzymes (Jones (1980) Ann. Rev. Biochem. 49:253-279). In many organisms, including man, the first three enzymes of de novo pyrimidine biosynthesis are found as multifunctional polypeptides. Typically, in higher eukaryotes the CPS, ATC, and DHO activities are encoded on a single gene, CAD, that specifies a single multifunctional polypeptide chain. In lower eukaryotes such as S. cerevisiae, the CAD-homologue gene specifies functional CPS and ATC domains, but a non-functional DHO domain. The organization of these CAD activities has evolved differently in various parasitic protozoa. In protozoan parasites of phylum Apicomplexa, including Babesia and Plasmodium species, the CPS activity is specified as an individual gene specifying a polypeptide with a single CPSII enzyme activity comprising the glutamine amido transferase (GAT) activity and the CPS activity; GAT+CPS=CPSH (Chansiri and Bagnara (1995) Mol. Biochem. Parasitol. 74:239-243; Flores, et al. (1994) Mol. Biochem. Parasitol. 68:315-318). This peculiar protozoan parasite gene organization is more similar to bacteria where CPS is monofunctional (Mergeay, et al. (1974) Mol. Gen. Genet. 133:299-316). T. gondii is also an apicomplexan and it also specifies the CAD enzyme activities on individual polypeptides (Asai, et al. (1983) Mol. Biochem. Parasitol. 7:89-100). This difference in CAD gene organization between man and apicomplexan parasites is reminiscent of the situation with DHFR and TS where these enzyme activities are present on a single polypeptide in apicomplexan parasites (Bzik, et al. (1987) Proc. Natl. Acad. Sci. USA 84:8360-8364) and on individual polypeptides in man. The difference in DHFR-TS gene structure between parasites and man has provided significant opportunity for chemotherapy using compounds such as pyrimethamine.

The difference in the CAD gene structure for pyrimidine synthesis between parasites and man also provides a unique chemotherapeutic opportunity. Further, blocking the accumulation of UMP by attacking one of the de novo pyrimidine biosynthetic enzymes should have a more profound anti-parasite effect than, for example, blocking accumulation of dTMP via pyrimethamine and sulfonamide treatment which is the standard chemotherapy for recrudescent toxoplasmosis. The latter strategy primarily blocks tachyzoite DNA replication with little apparent effect on bradyzoites, whereas the former strategy is predicted to block both parasite RNA synthesis as well as DNA replication.

In addition to the novel protozoan gene organization of CAD, the CAD-encoded enzymes have unique properties and regulation that make them attractive targets for chemotherapy. The CPSII activity detected in T. gondii is primarily involved in de novo pyrimidine biosynthesis based on substrate preference (Asai, et al. (1983) supra). While the mammalian CPSI involved in the Urea Cycle is activated by N-acetyl glutamate, the CPSII activity found in T. gondii is not affected by this treatment. The T. gondii CPSII activity is inhibited by UTP, suggesting a pyrimidine-controlled regulatory circuit. While the CPSII activity of man is activated by P-rib-PP, the T. gondii CPS activity is not. In contrast to CPSII, other enzymes of the de novo biosynthetic pathway were broadly characterized to behave similarly to their higher eukaryotic counterpart. The T. gondii CPSII appears to have markedly different properties from mammalian CPSII (Asai, at al. (1983) supra).

While T. gondii has a complete system for de novo pyrimidine biosynthesis, it only has a limited capacity to salvage pyrimidine bases. A biochemical survey of pyrimidine salvage enzymes supports the theory that all T. gondii pyrimidine salvage is funneled through uracil (Iltzsch (1993) J. Euk. Micro. 40:24-28). T. gondii has only three enzymes that are involved in salvage of pyrimidine nucleobases and nucleosides: cytidine/deoxycytidine deaminase, which deaminates cytidine and deoxycytidine; uridine phosphorylase, which catalyzes the reversible phosphorolysis of uridine, deoxyuridine, and thymidine; and uracil phosphoribosyltransferase (UPRT), which catalyzes the formation of UMP from uracil. The uridine phosphorylase and UPRT activities are the key salvage enzymes since pyrimidine salvage funnels all pyrimidine compounds first to uracil and then the UPRT activity yields UMP (Pfefferkorn (1978) Exp. Parasitol. 44:26-35; Iltzsch, (1993) J. Euk. Micro. 40:24-28). The limited T. gondii pyrimidine salvage pathway is not required for viability. Mutations that abolish UPRT activity are tolerated and are equally viable to wild-type parasites in vitro and in vivo (Pfefferkorn (1978) supra; Donald and Roos (1995) Proc. Natl. Acad. Sci. USA 92:5749-5753). Furthermore, there is no available evidence that T. gondii actually salvages any pyrimidine bases from the host cell under normal in vivo or in vitro growth conditions.

The nucleic acid molecule encoding T. gondii carbamoyl phosphate synthetase II (CPSII) has now been identified. Furthermore, an attenuated non-replicating uracil auxotroph strain, cps1-1, was generated which induces long lasting immunity against a low dose lethal challenge with hypervirulent T. gondii strain RH. The cps1-1-induced immunity was mediated by CD8⁺ T cells and an early rapid influx of Gr-1⁺ granulocytes and Gr-1⁺ CD68⁺ inflammatory macrophages were observed at the sight of inoculation. CD19⁺ B cells and CD4⁺ T cells infiltrated the site of inoculation 4 days after exposure to cps1-1. Unexpectedly, CD8⁺ T cells responded earlier than CD4⁺ T cells. Immunization with cps1-1 was marked by low early systemic IFN-γ, IL-12p40, and IL-12p70 production with higher levels of these inflammatory cytokines occurring locally at the site of cps1-1 inoculation. Advantageously, production of IL-12 and IFN-γ is the basis for protection against a large number of different intracellular pathogens. Moreover, the production of IL-12p70 is particularly relevant given that is thought that is protein is the single molecule that is needed to signal dendritic cells (DCs) to produce the optimum immune responses. As conventional adjuvants can not induce such strong and controlled levels of IL-12p70, cps1-1 is a desirable vector for delivering vaccine antigens. Given that the cps1-1 mutant induces immune responses solely to cps1-1 without the complication of dead host cells and host-derived inflammation, antigens expressed by the cps1-1 mutant can be delivered in a defined dose of non-replicating parasites.

Accordingly, the present invention relates to an isolated nucleic acid molecule encoding an apicomplexan CPSII protein and use thereof to generate an attenuated uracil auxotroph mutant which lacks a functional CPSII enzyme. Said attenuated uracil auxotroph mutant finds application as a vaccine against an apicomplexan as well as a delivery vector for exogenous proteins (e.g., antigens), thus resulting in a multivalent vaccine against the apicomplexan and the disease associated with the exogenous antigen.

An isolated nucleic acid molecule encoding an apicomplexan CPSII protein is intended to encompass nucleic acid molecules encoding CPSII protein from the parasitic Apicomplexa organisms, P. falciparum, B. bovis, T. gondii, T. cruzi, Theileriai annulata, and T. parva. It was found that T. gondii RH genomic DNA clones encoding CPSII exhibited significant sequence homology with amino acid residues 304-1400 of the T. cruzi CPSII protein. Alignment of the derived amino acid sequence of T. gondii CPSII as set forth in SEQ ID NO:2 was highest with the corresponding sequences of CPSII from the parasitic Apicomplexa organisms, P. falciparum, B. bovis, T. cruzi, Theileria annulata, and T. parva. Regions of amino acid sequence identity amongst the CPSII proteins of P. falciparum (GENBANK Accession No. CAD52216), B. bovis (GENBANK Accession No. AAC47302), T. gondii, Theileria annulata (GENBANK Accession No. CAI75289), T. parva (GENBANK Accession No. XP_(—)763067) and T. cruzi (GENBANK Accession No. BAA74521) included, Glu-Xaa-Asn-Ala-Arg-Leu-Ser-Arg-Ser-Ser-Ala-Leu-Ala-Ser-Lys-Ala-Thr-Gly-Tyr-Pro-Leu-Ala (SEQ ID NO:7), wherein Xaa is an aliphatic amino acid (i.e., Ile, Leu or Val); Met-Lys-Ser-Val-Gly-Glu-Val-Met-Xaa₁-Ile-Gly-Xaa₂-Thr-Phe-Glu-Glu-Xaa₃ (SEQ ID NO:8), wherein Xaa₁ and Xaa₃ are independently Ser or Ala and Xaa₂ is positively charged amino acid residue (e.g., Lys, Arg, or His); and Leu-Xaa₁-Arg-Pro-Ser-Tyr-Val-Leu-Ser-Gly-Xaa₂-Xaa₂-Met (SEQ ID NO:9), wherein Xaa₁ is Val or Ala and Xaa₂ is Ser or Ala. Accordingly, certain embodiments embrace a nucleic acid molecule encoding an apicomplexan CPSII protein having the amino acid sequence set forth in SEQ ID NO:7, SEQ ID NO:8, and SEQ ID NO:9. In other embodiments, the nucleic acid molecule encodes a CPSII protein from a Toxoplasma species. In one embodiment, the nucleic acid molecule encodes a CPSII protein as set forth in SEQ ID NO:2. In another embodiment, the nucleic acid molecule has the sequence as set forth in SEQ ID NO:1.

An attenuated uracil auxotroph mutant of an apicomplexan is defined herein as a mutant apicomplexan, which lacks a functional CPSII enzyme. The mutant apicomplexan can be generated using any suitable method. For example, the mutant can be obtained by the single cross-over integration approach disclosed herein or using a double-crossover gene replacement, e.g., as disclosed by Mercier, et al. ((1998) Infect. Immun. 66:4176-82). See also Wang, et al. (2002) Mol. Biochem. Parasitol. 123(1):1-10. In general, the generation of a mutant apicomplexan includes isolating the nucleic acid molecule encoding the CPSII enzyme from the apicomplexan of interest; replacing, mutating, substituting or deleting all or a portion (e.g., 1 bp to 27 kb) of the CPSII gene to disrupt the promoter (e.g., nucleotides 1-2429 of SEQ ID NO:1) regulatory sequence(s) and/or open reading frame of the CPSII enzyme (e.g., nucleotides 2430-26452 of SEQ ID NO:1); and integrating the disrupted molecule (e.g., via single- or double-crossover homologous recombination events) into the genome of the apicomplexan of interest. Upon selection, i.e., marker protein expression or genomic DNA analysis, an uracil auxotrophic mutant which lacks a functional CPSII enzyme is obtained. Disruption of all or a portion of the CPSII gene can be achieved by, e.g., replacing the CPSII coding sequence with a nucleic acid molecule encoding selectable marker, replacing the CPSII coding sequence with a nucleic acid molecule encoding an exogenous protein, replacing the CPSII coding sequence with a nucleic acid molecule encoding a mutant CPSII protein, substituting the CPSII promoter with a mutated CPSII promoter which can no longer be recognized by T. gondii transcription proteins, etc. In particular embodiments, the uracil auxotroph has one or more of the mutations in CPSII as set forth herein, including but not limited mutations at amino acid residues 171-229, 345, 348, 385, 435, 454-470, 533, 873-910, 1316, 1318, 1336, 1430, 1530, 1592-1628, or 1649 of SEQ ID NO:2.

An attenuated uracil auxotroph mutant which lacks a functional CPSII enzyme can also be achieved by replacing (e.g., by double-crossover gene replacement) the wild-type CPSII enzyme with a mutant CPSII enzyme having one or more point mutations at active site or ATP binding domains. Selection of residues to be mutated can be based upon, e.g., the crystal structure of CPS from E. coli (Thoden, et al. (1997) Biochemistry 36:6305-6316) or biotin carboxylase which has been shown to have functionally equivalent ATP binding domains with CPS enzymes (see, e.g., Kothe, et al. (1997) Proc. Natl. Acad. Sci. USA 94:12348-12353).

Given that the sequences obtained from the cDNA and gDNA clones of T. gondii CPSII indicate that T. gondii has a CPSII genomic locus organized like other apicomplexan CPSII loci, it is contemplated that methods used to produce the T. gondii mutants disclosed herein are applicable to the generalized construction of uracil auxotroph mutants in other apicomplexan parasites including, but not limited to, parasitic species of Theileria, Babesia, Plasmodium and other species of Toxoplasma (e.g., T. cruzi) which encode a CPSII enzyme containing the common amino acid sequences set forth in SEQ ID NOs:7-9. In one embodiment, the mutant apicomplexan is a species of Toxoplasma. In another embodiment, mutant apicomplexan is T. gondii.

As will be appreciated by the skilled artisan, any suitable marker-encoding nucleic acid can be used to identify a cpsII mutant so long as it can be phenotypically detected in the mutant strain. Suitable marker proteins include, but are not limited, positive and negative selectable markers such as thymidine kinase, hygromycin resistance, cytosine deaminase, DHFR, bleomycin, chloramphenicol acetyl transferase and the like. It is contemplated that the nucleic acid molecule encoding the marker protein can be used to replace or substitute all or a portion of the promoter or coding sequence of the CPSII locus to generate a mutant which lacks a functional CPSII enzyme.

A mutant is said to lack a functional CPSII enzyme when there is no detectable CPSII enzyme activity and the mutant is completely dependent on pyrimidine supplementation for replication (e.g., as determined by a plaque assay in the presence between 200 and 400 μM uracil). In this regard, a “leaky” mutant (i.e., a mutant which expresses a detectable amount of protein, exhibits a detectable amount of CPSII activity, or is not completely dependent upon pyrimidine supplementation for replication) is not encompassed within the definition of an apicomplexan mutant which lacks a functional CPSII enzyme.

Having demonstrated that an attenuated uracil auxotroph mutant of T. gondii provides protection against infection by parasitic T. gondii and induces a Th-1 immune response which is specific to T. gondii antigens without the complication of dead host cells and host-derived inflammation, the present invention also embraces the use of an attenuated T. gondii uracil auxotroph mutant for intracellular vaccination by delivering exogenous antigens from non-T. gondii disease agents (i.e., antigens not naturally expressed by the T. gondii). In one embodiment, the exogenous antigen is expressed by T. gondii, secreted into the parasite vacuole and eventually into the cytosol of the mammalian host cell. The T. gondii-expressed exogenous antigen subsequently enters the mammalian antigen presenting cell's (APC) antigen processing and presenting pathway as a substrate for generation of class I and class II peptides which generate CD8 and CD4 T cell responses. Accordingly, in one embodiment of the present invention, the attenuated T. gondii uracil auxotroph mutant harbors a nucleic acid molecule encoding an exogenous antigen. In this regard, the inventive T. gondii uracil auxotrophic can be used to vaccinate against both T. gondii and against any non-T. gondii antigen(s) encoded by gene(s) expressed in the T. gondii uracil auxotroph. This includes both protein antigens and also non-protein antigens that could be produced by genes within the T. gondii carrier, such as polysaccharides and lipids. While certain embodiment embrace the expression of antigens from pathogenic organisms, e.g., bacteria, fungi, viruses, and parasites, other embodiments include any other antigen to which an immune response would be desired, e.g., host antigens such as tumor antigens.

It is further contemplated that an attenuated or non-attenuated mutant of T. gondii can be used to express any other genes one would want to express within a mammalian host cell. This could include genes encoding therapeutic peptides or proteins, e.g., therapeutic antibodies (e.g., Trastuzumab) proteins (e.g., interferons, blood factors, insulin, erythropoietin, and blood clotting factors), or enzymes (e.g., asparaginase, catalase, lipase, and tissue plasminogen activator) used in the treatment of diseases or conditions; as well as proteins, enzymes or peptides of use in screening assays to identify inhibitors or activators (i.e., effectors) of the same. Such proteins are routinely expressed in other systems, e.g., yeast, mammalian cells lines, bacteria or insect cells, such that one skilled in the art could readily obtain nucleic acids encoding such proteins and express them in a mutant T. gondii.

The T. gondii uracil auxotroph of the present invention can accommodate multiple expression constructs. Therefore, nucleic acid molecules encoding exogenous antigens from a non-T gondii disease can be integrated into the T. gondii genome, e.g., as part of the nucleic acid molecule used to disrupt the promoter, regulatory sequences, or open reading frame of the CPSII enzyme or at any other suitable location in the genome (e.g., at non-essential loci). Examples of exogenous antigens include tetanus toxoid (tetC); malarial antigens such as circumsporozoite protein (CSP) and merozoite surface protein-1 (MSP-1); Bacillus anthracis protective antigen; Yersinia pestis antigens (e.g., Fl+V and Fl−V fusion); antigens from intracellular bacterial pathogens such as Francisella tularensis, Mycobacteria, Legionella, Burkholderia (e.g., B. pseudomallei and B. mallei), Brucella species and Coxiella; antigens from viruses, particularly intracellular invaders such as HIV; other toxoids such as botulinum toxoid or Epsilon toxin; tumor antigens; multiagent biodefense antigens; antigens from non-biothreat infectious agents; plague antigens; and combinations of any of these. As indicated above, it is also contemplated that exogenous genes encoding enzymes which synthesize non-protein antigenic products, e.g., lipids or polysaccharides, can be expressed in the T. gondii platform. Care should be taken to ensure that antigens being expressed in T. gondii are not functional virulence factors. Therefore, it may be desirable to use known protective antigens not representing virulence factors or use mutated genes that do not encode complete toxin or virulence factors.

The basic criteria for exogenous antigen expression are that the gene is a non-T. gondii gene or coding sequence and the gene or coding sequence is able to be expressed directly or indirectly from a recombinant molecule in a T. gondii cell. In this regard, it is desirable that the promoter employed is recognizable by T. gondii. Moreover, is desirable that the promoter promotes transcription of the antigen coding sequence when the T. gondii is inside mammalian cells. To this end, particular embodiments embrace the use of a T. gondii promoter. Known promoter and other regulatory elements (e.g., 5′ UTR, 3′ UTR, etc.) which can be operably linked to the coding sequence of an exogenous antigen of interest so that the exogenous antigen is expressed in T. gondii include, but are not limited to, sequences from the T. gondii SAG1 gene (Striepen, et al. (1998) Mol. Biochem. Parasitol. 92(2):325-38) or the T. gondii NTPase gene (Robibaro, et al. (2002) Cellular Microbiol. 4:139; Nakaar, et al. (1998) Mol. Biochem. Parasitol. 92(2):229-39). Alternatively, suitable regulatory sequences can be obtained by known trapping techniques. See, e.g., Roos, et al. (1997) Methods 13(2):112-22. Promoters of use in accordance with the present invention can also be stage-specific promoters, which selectively express the exogenous antigen(s) of interest at different points in the obligate intracellular T. gondii life cycle. Moreover, it is contemplated that an endogenous promoter can be used to drive expression of the exogenous antigen by, e.g., site-specific integration at the 3′ end of a known promoter in the T. gondii genome.

One advantage of such multivalent vaccines is that protection against multiple disease agents can be attained with a single vaccine formulation. The protective immune response to T. gondii uracil auxotroph clearly involves both a humoral (antibody) response and the cell-mediated component of immunity, thus a diverse immune response to any expressed antigen is possible. In this regard, the instant T. gondii-based vaccine can serve as an agent of protection and of adjuvancy for any exogenous antigen(s) expressed.

When employed as a vaccine for protection against infection by T. gondii and/or a non-T. gondii disease, particular embodiments provide that the attenuated T. gondii uracil auxotroph mutant is in admixture with a pharmaceutically acceptable carrier. Suitable carriers and their formulation are described, for example, in Remington: The Science and Practice of Pharmacy, Alfonso R. Gennaro, editor, 20th ed. Lippincott Williams & Wilkins: Philadelphia, Pa., 2000. Furthermore, it has now been shown that ex vivo loading of dendritic cells, macrophages, and peritoneal cells with cps1-1, then immunizing an animal with these loaded cells leads to successful immunization. Dendritic cells loaded with cps1-1 provide the strongest immune response in animals and produce long lasting CD8 T cell responses and long lasting immune memory. Accordingly, it is contemplated that the attenuated T. gondii uracil auxotroph or vaccine of the same can be administered via loading of dendritic cells, macrophages, and/or peritoneal cells.

Administration of a composition disclosed herein can be carried out by any suitable means, including parenteral injection (such as intraperitoneal, subcutaneous, or intramuscular injection), orally, or by topical application (typically carried in a pharmaceutical formulation) to an airway surface. Topical application to an airway surface can be carried out by intranasal administration (e.g., by use of dropper, swab, or inhaler which deposits a pharmaceutical formulation intranasally). Oral administration can be in the form of an ingestible liquid or solid formulation. In particular embodiments, administration is via intraperitoneal or intravenous routes.

An attenuated T. gondii uracil auxotroph or vaccine containing the same can be employed in various methods for protecting a subject against infection by T. gondii and/or a non-T. gondii disease. Such methods generally involve administering to a subject in need of treatment (e.g., a subject at risk of being exposed to an infectious disease or at risk of developing cancer) an effective amount of a T. gondii uracil auxotroph or vaccine of the present invention thereby protecting the subject against infection by T. gondii and/or the non-T. gondii disease. An effective amount, as used in the context of the instant invention, is an amount which produces a detectable Th-1 immune response or antibody response to an antigen thereby generating protective immunity against the pathogen or disease from which the antigen was derived or associated. As such, an effective amount prevents the signs or symptoms of a disease or infection, or diminishes pathogenesis so that the disease or infection is treated. Responses to administration can be measured by analysis of subject's vital signs, monitoring T cell or antibody responses, or monitoring production of IFN-γ, IL-12p40, and/or IL-12p70 according to the methods disclosed herein or any suitable method known in the art.

Administration can be given in a single dose schedule, or a multiple dose schedule in which a primary course of treatment can be with 1-10 separate doses, followed by other doses given at subsequent time intervals required to maintain and or reinforce the response, for example, at 1-4 months for a second dose, and if needed, a subsequent dose(s) after several months.

The exact dosage for administration can be determined by the skilled practitioner, in light of factors related to the subject that requires prevention or treatment. Dosage and administration are adjusted to provide sufficient levels of the composition or to maintain the desired effect of preventing or reducing signs or symptoms of the disease or infection, or reducing severity of the disease or infection. Factors which may be taken into account include the severity of the disease state, general health of the subject, age, weight, and gender of the subject, diet, time and frequency of administration, drug combination(s), reaction sensitivities, and tolerance/response to therapy.

While the instant composition and methods find application in the prevention and treatment of diseases or infections of mammals, in particular humans, the invention should be construed to include administration to a variety of animals, including, but not limited to, cats, dogs, horses, cows, cattle, sheep, goats, birds such as chickens, ducks, and geese. In this regard, the instant invention is also useful against potential bioterrorism aimed at agriculture and populace, e.g., Brucella and anthrax. As such, the instant T. gondii vector platform can be employed by both pharmaceutical and agribusiness to produce multivalent vaccines with intracellular delivery to create commercial multiagent vaccines for people and livestock. The disclosed T. gondii vector platform also can be employed more broadly in the development of new treatments or preventions (vaccines) against a wide variety of human diseases such as cancer, autoimmune disorders, vascular disorders, and musculoskeletal disorders.

Moreover, as the demonstrated herein, there are particular regions of the T. gondii CPSII which can be used in screening assays to identify parasite-selective drugs for preventing or treating a T. gondii infection. For example, deletion of the GATase indel as well as the C-terminal regulatory indel completely abolished complementation activity of a CPSII minigene showing these indels to be necessary for CPSII function and parasite growth in vivo. Furthermore, the results presented herein demonstrate a functional and essential role for a number of C-terminal amino acids known to transmit the allosteric regulatory signals in E. coli CPS or mammalian CPSII. T. gondii CPSII is dependent on GATase for production of ammonia in vivo, and an ammonia tunnel potentially analogous to the described ammonia tunnel in E. coli CPS is present. Therefore, these results reveal unique features of T. gondii CPSII that provide parasite-selective drug targets to inhibit CPSII thereby starving the parasite of essential pyrimidines required for RNA and DNA synthesis. Thus, conventional in vitro or in vivo drug screening assays can be employed to identify agents which modulate the activity of CPSII by targeting one or more of the essential amino acid residues or regions of T. gondii CPSII disclosed herein. Agents which can be screened in accordance with such an assay can include compounds such as antibodies, proteins, peptides, nucleic acids, small organic molecules, etc.

In so far as mutant T. gondii of the present invention can be modified to express any protein of interest, particular embodiments of this invention feature the use of a transgenic T. gondii strain expressing a human therapeutic target protein or enzyme in live, whole-cell screening assays for identifying effectors (i.e., inhibitors or activators) of said target protein or enzyme. Such whole-cell screening assays are routinely practiced in the art using other systems, e.g., yeast, mammalian cell lines, or bacteria, and can be readily adapted for use with T. gondii-infected cells. For the purposes of the present invention, a human therapeutic target protein or enzyme is any human protein, which is known to be involved in the manifestation of a disease or condition. In this regard, inhibition or activation of the human therapeutic target protein or enzyme results in the prevention or treatment of the disease or condition. Human therapeutic target proteins or enzymes are well-known to those skilled in the art.

The addition to the above-described uses, the mutants of this invention also find application as “sensors” of the intracellular mammalian cell environment. The cellular events inside of living mammalian cells during drug treatments for cancer, autoimmune disease and any other disease state or treatment state is typically difficult to measure, but is of major commercial interest. Many conventional treatments work by starving cells of pyrimidines or purines, or by adding pyrimidines (5-fluorouracil for cancer) or purines (mercaptopurine for cancer) derivatives. In this regard, uracil auxotroph mutants of the invention can be used to sense the concentration of pyrimidines and pyrimidine analogs present in the host cell.

For example, the pyrimidine auxotrophs disclosed herein rely on host cell uracil and uridine (and deoxyuridine) for growth. Thus, these mutants can both “sense” the host cell environment for these compounds measured by parasite growth rate, and also “sense” cytotoxic concentrations of 5-flurouracil or 5-fluorouridine (or deoxyuridine) concentrations based on stasis and/or death of the intracellular parasite.

The following non-limiting examples are provided to further illustrate the present invention.

Example 1 Materials, Assays, and Molecular Methods

Parasitic Material. Most of the work described herein used the RH strain of Toxoplasma gondii which is the most commonly used laboratory strain amongst Toxoplasma researchers (Pfefferkorn, et al. (1976) Exp. Parasitol. 39:365-376). Due to its long history of continuous passage in the laboratory, this strain is highly virulent in animals and grows rapidly in culture making it ideal for obtaining large amounts of material. However, it has lost the ability to go through the complete sexual cycle in cats.

Tachyzoites of the virulent Type I strain RH and the high-passage in vitro cultivated Type II parasite PLK were obtained by serial passage in human foreskin fibroblast (HFF) cell monolayers in EMEM, 1% FBS at 37° C. in 95% air/5% CO₂. Typically, infected cultures were maintained by seeding uninfected monolayers at about a 1:50 dilution every 48-72 hours. This yielded about 10⁹ parasites from three T175 flasks of infected cultures.

Tachyzoites were isolated from freshly lysed HFF monolayers by filtration through 3.0 μm nuclepore membranes, washed with phosphate-buffered saline (PBS), centrifuged, and then resuspended in PBS at defined numbers. Viability of tachyzoite preparations was tested in plaque assays to confirm that 30-50% of tachyzoites were infectious.

Detailed methods for growth, harvesting, passage, purification of tachyzoite parasites, storage, and replication assays of T. gondii are routine and well-known, for example, Roos, at al. (1994) Meth. Microb. Path. 45:23-65; Fox, et al. (1999) supra. All media reagents were purchased from GIBCO-BRL (Rockville, Md.), Bio Whittaker (Walkersville, Md.) and Sigma (St. Louis, Mo.).

Chemicals and Enzyme Assays. Most chemical or biochemical reagents were purchased from Sigma (St. Louis, Mo.). Ganciclovir was obtained from Roche Labs (Nutley, N.J.). The linked assay system for CPSII activity was performed in accordance with known methods, for example, Asai, et al. (1983) Mol. Biochem. Parasitol. 7:89-100 and Hill, et al. (1981) Mol. Biochem. Parasitol. 2:123-134. For CPSII assays parasites were lysed in M-PER extraction buffer (Pierce Inc., Rockford, Ill.) or by osmotic shock in 4 volumes (w/v) of 10 mM potassium phosphate (pH 7), 0.05 mM dithiothreitol and protease inhibitors antipain, leupeptin, chymostatin, and pepstatin A, each at 0.1 mM. After 1 to 2 minutes of lysis, glycerol 7.5% (w/v) was added to the extracts. The lysed parasite extracts were centrifuged at 20,000×g for 15 minutes and the supernatants used in CPSII enzyme assay. CPSII reaction assays contained 50 mM HEPES (pH 7.2), 10% (w/v) glycerol, 20 mM MgCl₂, 20 mM ATP, 3 mM L-glutamine, 0.5 mM L-ornithine, 10 mM KCl, 0.05 mM dithiothreitol, 1 unit ornithine carbamyl transferase, 10 mM bi[¹⁴C]carbonate (1 μCi/μM) and extracted in a final volume of 0.1 ml. Sodium bi[¹⁴C]carbonate was 30-60 mCi/mmol and obtained from ICN. Reactions were run for 30 minutes at 37° C. and then terminated by addition of 10 μl of 5 M formic acid. A small piece of solid CO₂ was dropped into the stopped reaction and excess CO₂ was removed by standing the open solution in a fume hood. Reaction volumes were removed, dried and redissolved in 0.1 ml water prior to addition of liquiscint scintillant and counting of [¹³C] in a Beckman scintillation counter.

Thymidine kinase assays were performed in accordance with known methods, for example, Maga, at al. ((1994) Biochem. J. 302:279-282). Briefly, parasite extracts were lysed by sonication (1 minute, kontes microtip) or in M-PER extraction buffer plus a cocktail of protease inhibitors (antipain, leupeptin, chymostatin, and pepstatin A; 10 μg each) and 1 mM phenylmethylsulfonyl fluoride. TK assays were run in a 50 μl volume at 37° C. for 30 minutes in a mixture containing 30 mM potassium HEPES (pH 7.5), 6 mM ATP, 6 mM MgCl₂, 0.5 mM dithiothreitol and 3.3 μM [³H]Thymidine (20-40 Ci/mmol from ICN). The reaction was terminated by transferring 25 μl of the incubation mixture to DE81 ion exchange paper (Whatman, Clifton, N.J.). The spotted paper was washed in 1 mM ammonium formate (pH 3.6) to remove unconverted nucleoside, distilled water, and then a final ethanol wash prior to drying of the paper and scintillation counting in liquiscint. Protein determination for parasite protein extracts was determined using BIO-RAD protein assay reagents and bovine serum albumin in accordance with standard procedures (BIO-RAD, Hercules, Calif.).

Molecular Methods. Molecular methods including DNA isolation, restriction, Southern blot analysis, hybridization, and PCR reactions used herein are all well-known, for example, Bzik, at al. (1987) Proc. Natl. Acad. Sci. USA 84:8360-8364 and Fox, et al. (1999) supra. Transfection of T. gondii was also performed in accordance with routine procedures (Roos, et al. (1994) supra and Fox et al. (1999) supra). The gene libraries were developed from HindII- or PstI-digested genomic DNA cloned into BLUESCRIPT KSII digested with the same enzyme and treated with alkaline phosphatase prior to ligation with T. gondii DNA fragments. Libraries were manipulated in accordance with known methods (Bzik, et al. (1987) supra). Total mRNA was isolated from T. gondii using TRIZOL-LS reagent (GIBCO-BRL, Rockville, Md.) and mRNA was converted to cDNA using a cDNA kit from Pharmacia (Piscataway, N.J.) with polydT or random hexamers primers. DNA sequencing was done using classical dideoxy chain termination or automated sequencing using fluorescent dyes (ABI sequencer, Foster City, Calif.). DNA sequences were analyzed using the MACVECTOR suite of programs (Oxford Molecular, Beaverton, Oreg.) and resources at NCBI, such as blast search. The DHFRm2m3-TS allele was obtained from the NIH AIDS Reference and Reagent Center (Rockville, Md.). The TK75 allele which was described by Black, et al. (1996) supra was obtained from Darwin (Seattle, Wash.). BLUESCRIPT plasmid was from STRATAGENE (La Jolla, Calif.). Restriction enzymes, nucleic acid modifying enzymes and transfer membranes were from Boehringer Mannheim, Indianapolis, Ind.

Flow Cytometric Differential Cell Analysis. Peritoneal excudate cells (PECS) from uninfected wild-type mice for day 0 controls and infected wild-type mice on days 2, 4, 6, and 8 post-infection were obtained by peritoneal lavage with 5-7 ml of ice-cold PBS. PECS were depleted of erythrocytes by treatment with buffered ammonium chloride (Obar, et al. (2004) J. Immunol. 173:2705-2714), washed in PBS, then counted to determine total viable cell numbers recovered by trypan blue exclusion. Differential cell analysis via flow cytometry was then performed using either single or dual color staining of 2-5×10⁵ PECS per animal, PECS were stained using standard procedures in SWB buffer containing 2% FBS in 1×PBS and 5% normal mouse serum (SIGMA, St. Louis, Mo.) and Mouse FC Block (BD Biosciences, San Jose, Calif.) to block Fc receptors. Single-cell suspensions were initially fixed in SWB 1% paraformaldehyde then stained in primary reactions for either the surface markers PE-conjugated anti-mouse CD3 (17A2) or anti-mouse Gr-1 (RB6-8C5) alone or with one of each of the following FITC-conjugated antibodies anti-mouse CD45R/B220 (RA3-6B2), anti-mouse CDllb (Ml/70) (BD Biosciences), anti-mouse CD19, anti-mouse F4/80, or the intracellular stain anti-mouse CD68 (Serotec, Raleigh, N.C.). For the intracellular stain (CD68), fixed PECs were permeabilized using SWB 0.5% saponin then stained for anti-mouse CD68 using SWB 0.5% saponin as stain buffer. Flow cytometric data were acquired and analyzed using the FACS CALIBUR flow cytometer and CELLQUEST software (BD Immuncytometry Systems, San Jose, Calif.). Percentage of total events were based on 2-5×10⁴ total events per sample and are the sum of the gates R1, R2 and R3. Granulocyte percentages were calculated from Gr-1⁺ CD68⁻ events in R3. Each percentage was verified by total event analysis. Absolute numbers were the product of percentage of total events multiplied by total cell number per mouse. Non-specific Ig isotype specific antibodies were used as negative controls for all samples (BD Biosciences).

Cytokine Assays. The concentrations of the mouse cytokines were measured by ELISA in the serum and in vitro culture supernatants of whole PECs or splenocytes harvested from wild-type mice injected i.p. with either 1×PBS, 1×10⁶ cps1-1 tachyzoites at various times post-infection. Serum was obtained from whole blood by incubation at room temperature for 2 hours and coagulated blood was centrifuged for 10 minutes at 14,000 rpm at 4° C. and serum was frozen at −80° C. Immediately after obtaining blood, mice were euthanized via CO₂ overdose then PECS and spleen harvested. Single cell suspensions of splenocytes and PECs were depleted of RBCs and counted as described above, then resuspended in DMEM with 10% FBS and 1× antimicrobe/antimycotic (GIBCO BRL). Cells were seeded in 24-well trays at either 1×10⁶ or 5×10⁶ PECs or splenocytes, respectively, per well and cultured for 24 hours at 37° C., 5% CO₂ (Mordue & Sibley (2003) J. Leukoc. Biol. 74:1015-1025). Cells in 24-well trays were freeze-thawed 3 cycles (−80° C. to +37° C.), debris was removed from the supernatants by centrifugation at 14,000 rpm at 4° C. for 10 minutes and the supernatants were stored at −80° C. Concentrations of the mouse cytokines IFN-γ, IL-12p40, and IL-12p70 were then determined using OPTELA ELISA sets (BD Biosciences) (Robben, et al. (2004) J. Immunol. 172:3686-3694) following the manufacturer's instructions. Serum was used at a 1:4 dilution in assay diluent whereas supernatant from PECs or splenocytes was not diluted.

Tg Lysate Preparation and Serum IgG Assay. Total tachyzoite lysate antigen was prepared from in vitro HFF cultures of RH (Nguyen, et al. (2003) Magn. Reson. Med. 50:235-241). Tachyzoites were purified by nucleopore filtration, counted then pelleted at 1500 g for 6 minutes at 4° C. Tachyzoites were washed once, resuspended in 1× PBS, then disrupted by sonication on ice four times for 20 seconds (Gazzinelli, et al. (1991) J. Immunol. 146:286-292; El-Malky, et al. (2005) Microbiol. Immunol. 49:639-646). TLA was then filtered through a 0.22 μm pore filter and stored at −80° C. Anti-toxoplasma-specific serum IgG responses were measured in cps1-1 immunized mice by coating each well of a 96-well plate with 100 μl of TLA at a concentration of 10 μg/ml in 1×PBS overnight at 4° C. (El-Malky, et al. (2005) supra). Plates were washed three times with 1×PBS then blocked with 1×PBS/1% BSA. Sera were serially diluted three-fold in PBS 0.1% BSA and applied to each well in triplicate then plates were incubated overnight at 4° C. Plates were washed with PBS to remove non-specifically bound Ig and secondary anti-mouse Ig-specific HRP-conjugated antibody was used to detect the toxoplasma-specific antibodies. Conjugated secondary Ig was either anti-mouse IgG H+L or isotype-specific anti-IgG1 and IgG2a. TMB substrate was used to detect secondary antibodies followed by stopping the reaction with 0.2N H₂SO₄ (Bourguin, et al. (1998) Infect. Immun. 66:4867-4874). Plates were then read at 405 nm on an ELISA plate reader.

Adoptive Transfer and T Lymphocyte Antibody Depletion. For adoptive transfers, spleens were harvested three weeks after final immunization with 1×10⁶ cps1-1 tachyzoites, and used as a source for either toxoplasma-specific memory T or B lymphocytes. Spleens were disrupted immediately after removal from mice by grinding between microscope slides, pelleting, then depleting RBCs as described. CD19⁺, CD8⁺, or CD4⁺ T cells were purified using EASYSEP Mouse positive selection kits and purity was confirmed as per manufacturer's instructions (StemCell Technologies Inc.) (Obar, et al. (2004) supra). Naive recipient wild-type mice were given either 1×10⁷ CD8⁺ T cells, 1×10⁶ CD4⁺ T cells, 1×10⁶ CD4⁺ and 1×10⁶ CD8⁺ T cells, or 5.0×10⁶ CD19⁺ B cells via tail vein injection. Twenty-four hours after transfer of purified cell populations, mice were high dose i.p.-challenged with 1×10³ RH tachyzoites (Ely, et al. (1999) J. Immunol. 162:5449-5454). To deplete specific T cell populations immunized mice were injected i.p. with 500 μg of either control Rat IgG, anti-CD4 antibody (GK1.5), anti-CD8 antibody (TIB210), or both anti-CD4/CD8 antibody on days −3, −2, −1, 0 then every other day until day 21 post-challenge (Obar, et al. (2004) J. Virol. 78:10829-10832). On Day 0 of antibody treatment, T cell depletion was continued by flow cytometry of peripheral blood. Percent survival was measured of groups of four mice per condition and experiments were repeated twice.

Statistical Analysis. The Kaplan-Meier product limit test was used to measure significant differences between survival curves of i.v. and ex vivo loaded cell type route experiments (GRAPHPAD PRISM software). All other samples were subject to a Students t-test and are represented as the mean±SEM.

Example 2 Isolation of Gene Encoding CPSII

The DNA encoding T. gondii carbamoyl phosphate synthetase II (CPSII) has now been cloned. To clone the CPSII gene from the RH strain of T. gondii, a forward degenerate primer: CCN YTN GGN ATH CAY CAN GGN GAY (SEQ ID NO:3) and a reverse degenerate primer: YTC YTC MAA NGT YCT NCC KAT NGA CAT NAC (SEQ ID NO:4), were designed from two stretches of amino acid sequence, Pro-Leu-Gly-Ile-His-Thr-Gly-Asp-Ser-Ile (SEQ ID NO:5) and Gly-Glu-Val-Met-Ser-Ile-Gly-Arg-Thr-Phe-Glu-Glu (SEQ ID NO:6), respectively, which were well-conserved in the CPSII domain from various species. With these two primers, PCR amplification of strain RH single stranded cDNA derived from RH mRNA was performed in accordance with known procedures (Fox, et. al (1999) Mol. Biochem. Parasitol. 98:93-103). A PCR product of the expected length, PCR 450 bp (see FIG. 1) was obtained. The 450 bp amplicon was excised from agarose, purified and cloned into the SS phage vector M13 mp9 in both orientations, for single-stranded sequencing using the dideoxy termination method.

The purified amplicon was then reamplified and used to probe lambda phage cDNA libraries from the NIH AIDS Reference and Reagent Center and a 1.2 Kb cDNA phagemid clone (pCPSII lc-1) was identified and transduced into BLUESCRIPT plasmid (STRATAGENE) for analysis via the manufacturer's protocol. A 1.0 Kb EcoRI fragment from pCPS lc-1 was shot-gun cloned into M13 mp9 and SS dideoxy sequenced. The sequences were found to align to those of the original 450 bp M13 mp9 clone and to have high homology to CPSII of other species. Separately, Southern blot analysis of T. gondii genomic restriction digests were probed with the gel-purified 450 bp fragment. This probe hybridized to several restriction fragments derived from RH parasite DNA including a unique band generated by Hind=(6.5 Kb) (see FIG. 1). Genomic libraries were then constructed in BLUESCRIPT SKII* phagemid vector that would contain the 6.5 Kb HindIII fragment and these genomic libraries were screened with the labeled 450 bp PCR-derived CPSII cDNA. Positive clones containing the desired insert in both orientations were isolated. The ends of TgH 2-11 6.5 Kb clone were then dideoxy double-strand sequenced using T3 and T7 primers. Primers from the ends of the sequenced sections were used to sequence the remainder. The 6.5 Kb HindIII fragment was also used to screen additional T. gondii genomic Southern blots and three PstI fragments were identified. Subsequently, a PstI T. gondii genomic library was constructed using standard methods and probed with fragments from either end of TgH 211 plasmid to yield 7.5 Kb, 4.9 Kb, and 3.8 Kb PstI clones that matched the corresponding sizes on the Southern blot. Locations of these clones within the genomic DNA and cDNA of T. gondii are depicted in FIG. 1.

The remainder of the CPSII genomic DNA clones were sequenced using a “walking” primer approach (see FIG. 1). For example, fragments of DNA at the 5′ end of clone 22-6-1 and the 3′ end of clone 18-7-1 (see FIG. 1) were used to identify additional fragments on Southern blot to obtain appropriate clones encoding the full genomic CPSII coding region plus flanking regulatory sequences. The full length genomic DNA sequence of T. gondii CPSII locus is set forth herein as SEQ ID NO:1. The coding sequence for T. gondii CPSII is obtained by joining nucleotides 2430 to 2487, 3053 to 3157, 3626 to 3732, 4323 to 4479, 5607 to 5962, 6417 to 6521, 7019 to 7125, 7754 to 7860, 8051 to 8131, 9015 to 9131, 9934 to 10062, 10519 to 10667, 11403 to 11492, 11860 to 11943, 12274 to 12433, 12698 to 12777, 13222 to 13333, 13944 to 13984, 14240 to 14371, 14907 to 14994, 15288 to 15381, 15875 to 15991, 16224 to 16615, 17103 to 17495, 17948 to 18045, 18484 to 18578, 19356 to 19587, 20384 to 20545, 21071 to 21207, 21930 to 21979, 22441 to 22570, 22860 to 23004, 23374 to 23476, 23844 to 23924, 24759 to 25001, 25582 to 25677, and 26322 to 26452 of SEQ ID NO:1.

Mutant T. gondii were also prepared wherein CPSII activity was knocked out. Knock out of this enzymatic activity or any other de novo pyrimidine synthesis enzyme was expected to produce a pyrimidine auxotroph that would be attenuated in mammals due to the inability of mammalian cells to provide the abundant pyrimidines needed by the parasite for growth. However, salvaging the growth of T. gondii purely by feeding pyrimidine compounds to the parasite in the growth medium was uncertain. Thus, pyrimidine salvage in T. gondii was examined.

Initial experiments primarily involved an enzymatic analysis of drug resistant mutants and the incorporation of various pyrimidine analogs into T. gondii RNA and DNA as an indication of pyrimidine salvage when parasites were grown in either normal host or mutant host cells. All biochemical communications between the parasite and host cells cross the vacuolar membrane which is now known to contain “pores” that permit the passage of nucleobases ranging in molecular mass from 112 daltons up to 244 daltons. The size of the pores is estimated to be approximately 1500 daltons. A T. gondii mutant resistant to 5-fluorodeoxyuridine (FUDR-1) had lost uracil phosphoribosyltransferase (UPRT), an enzyme which is absent in normal host cells (Pfefferkorn (1978) Expt. Parasitol. 44:26-35). Labeling of wild-type parasites or FUDR-1 parasites with [³H]deoxyuridine, [³H]uridine, and [³H]uracil revealed a striking pattern of pyrimidine incorporation into host or parasite nucleic acids. [³H]deoxyuridine was incorporated into wild-type T. gondii and labeled the host cell nucleus (DNA only since deoxyuridine is mainly converted into TTP by host cell enzymes). FUDR-1 mutant parasites were not labeled with [³H]deoxyuridine. [³H]uridine was incorporated into wild-type parasites and labeled host DNA (nucleus) and host RNA (cytoplasm) (uridine is incorporated into the host cell UTP pool by host cell UTP pool by host enzymes). FUDR-1 mutant parasites were not labeled with [³H]uridine. [³H]uracil was incorporated into wild-type T. gondii and did not label either host DNA or RNA. FUDR-1 (UPRT knock-out) parasites were not labeled with [³H]uracil.

In addition to the labeling patterns observed, wild-type RH parasites did not incorporate labeled orotic acid, orotate, cytosine, cytidine, thymine, or thymidine nucleobases. These results indicate that none of the host pyrimidine nucleotide pool is available to the parasite. Similarly, since uracil only labels the parasite, due to the parasite UPRT, which is absent in the host, the pyrimidine pools of the parasite are also unavailable to the host. Thus, there is no detectable pyrimidine traffic detected between the parasite and host.

Experiments to evaluate the feasibility of constructing pyrimidine auxotrophs of T. gondii were performed. Pyrimidine auxotrophy relies on the ability to feed mutant parasites a pyrimidine nucleobase, such as uracil, in culture medium in amounts that will restore parasite growth to near normal levels. Experiments were therefore conducted to measure whether uracil incorporation into parasites in culture could account for normal replication and normal growth rates. In these experiments, biochemically saturating amounts of [³H]uracil (25 μg/ml) were added to the growth medium and the quantitative incorporation of label into parasite RNA and DNA was determined over a four hour interval. It was calculated using known values that about 82% of the pyrimidines incorporated into parasite nucleic acids were derived from the uracil which was added to the growth medium as a supplement.

The uracil incorporation assay, results and calculation of uracil incorporation were carried out as follows. Parasites (2.09×10⁷) were labeled for 4 hours in medium containing [³H]uracil at 25 μg per ml or 0.223 μmole per ml, wherein the specific activity of uracil was 34.3 CPM per pmole (CPM=counts per minute). The results of this analysis indicated that the parasite culture incorporated 2.27×10⁵ CPM in 4 hours based upon the following parameters: 1) actual incorporation per parasite 0.11 CPM per tachyzoite=0.00032 pmoles pyrimidine per tachyzoite per 4 hours; 2) tachyzoites contain 0.10 pg DNA per cell; 3) tachyzoites contain 0.50 pg nucleic acid per cell (RNA:DNA ratio is 4:1); 4) Toxoplasma nucleic acids contain 112:760 pyrimidine by weight, thus, tachyzoites contain 0.50×0.1476=0.074 pg of pyrimidines; 5) pyrimidine molecular weight=112; 6) tachyzoites contain 0.074 pg/112/pg/pmole=0.00066 pmoles pyrimidine in nucleic acid; 7) tachyzoites double in 6 hours representing a 1.6-fold increase in the 4 hour labeling period, which equals the synthesis of 60% of nucleic acids in 4 hours; 8) theoretical 100% incorporation=60% of 0.00066 pmoles=0.00039 pmole pyrimidine incorporated per tachyzoite per 4 hours; and 9) actual incorporation (0.00032 pmoles) therefore represents 82% of the theoretical maximum incorporation (0.00039 pmoles). Based upon these parameters, it was concluded that 82% of incorporated pyrimidines originate from uracil when it is added to medium.

An 82% efficiency of incorporation of exogenously added uracil into parasite nucleic acid was detected. However, the same pathway mediating this incorporation (UPRT) can be completely abolished with no effect on parasite growth rates in vitro or in vivo. These results indicate that the parasite may activate the uracil salvage pathway to obtain near normal levels of uracil directly from growth medium when it is available or when uracil is needed, or alternatively to completely rely upon de novo biosynthesis when uracil is unavailable extracellularly. These biochemical measurements supported the feasibility of constructing stable pyrimidine auxotrophs of T. gondii by constructing a knock out of a gene and enzyme activity of the de novo pathway.

Example 3 Experimental Infection and Animal Studies

Animal Studies. Adult 6-8 week old C57BL/6 and C57BL/6 B cell-deficient mice (μMT) (Kitamura, et al. (1991) Nature 350:423-426) were obtained from Jackson Labs Bar Harbor, ME) as were Balb/c inbred mice and balb/c mice bearing a homozygous knock-out of interferon gamma (gko). Mice were maintained in Tecniplast Seal Safe mouse cages on vent racks. μMT mice were maintained in sterile conditions.

Tachyzoite parasites were aseptically handled and purified from freshly lysed monolayers of infected HFF cells through a sterilized 3 micron polycarbonate membrane (Nucleopore, Cambridge, Mass.). Parasite concentration was scored microscopically in a hemocytometer. Purified parasites were pelleted at 1500 g for 10 minutes and washed in sterile EMEM media with no supplements and without disturbing the parasite pellet. The centrifuge tube was centrifuged once more for 2 minutes and the supernatant removed and replaced with EMEM media containing no supplements in a volume of EMEM to give a 10 times higher concentration (per/ml) of parasites than the highest dose. This was done so inoculation of 0.1 ml of this solution would equal the highest parasite dose. Parasites were gently resuspended in sterile EMEM (no additions).

Mice were immunized with 1×10⁶ cps1-1 tachyzoites i.p, s.c, or i.v. once respectively or twice 14 days later with the same tachyzoite dose. At indicated times following the last immunization, mice were challenged with either low 1×10² or high (1×10³ or 1×10⁴) doses of viable RH or PLK Tachyzoites i.p. (Villegas, et al. (1999) J. Immunol. 163:3344-3353).

Following inoculation of mice the residual volume of unused tachyzoite parasites was returned to the sterile hood and dilutions were made to represent 200 and 400 parasite plaques on 25 cm² HFF flasks assuming 100% recovery of parasites after centrifugation/resuspension and 100% percent viability. Then, following a 7 day plaque assay, actual plaques were counted, post-inoculation of mice, and the percent viable PFU ratio to parasite counts in the hemocytometer were determined microscopically in every experimental infection. Uniformly, all of the mutants described herein as well as RH parasites always fell in the range of 0.4 to 0.6 viable PFU per parasite counted using these conditions. Following inoculation of mice, mice were observed daily for signs of infection (or distress) or death.

Ex vivo Infection. Dendritic cells (DCs) were obtained from the spleens of wild-type mice and purified using EASYSEP CDllc positive selection per the manufacturer's instructions. Briefly, spleens were harvested and injected with 1-2 ml of DNAse I/Liberase CI (Roche, Indianapolis, Ind.) followed by incubation at 37° C. for 30 minutes. Spleens were then ground through a 70-μm mesh nylon strainer and collected. DCs were then purified by CDllc magnetic positive selection and purity was verified as per manufacturer's instructions (StemCell Technologies Inc., Vancouver, BC). PECs were obtained from naïve mice and from a portion of those cells peritoneal-derived macrophages were obtained. PECs were plated at 4×10⁶ cells/ml in DMEM with 10% FBS and 1× antimicrobe/antimycotic and incubated for 4 hours at 37° C. Non-adherent cells were physically removed by washing gently with medium and remaining adherent cells were examined for macrophage purity via flow cytometry for percentage of CDl1b+ cells (>90%) (Da Gama, et al. (2004) Microbes Infect. 6:1287-1296). The DCs, PECs, and PEC-derived macrophages obtained were plated at 2×10⁶ cells/ml in infection medium consisting of EMEM with 1% FES, 1× antimicrobic/antimycotic and supplemented with 250 nM uracil (SIGMA, St. Louis, Mo.). Purified cps1-1 tachyzoites were inoculated into the wells containing specific cell populations at 5×10⁵ parasites/ml and infected cultures were incubated for 12 hours at 37° C. Infected cells were examined by light microscopy and at the time of harvest typically contained 4-8 cps1-1 tachyzoites. DCs, macrophages and PECs were washed to remove any residual extracellular tachyzoites, harvested and resuspended in PBS at 5×10⁵ cells/ml followed by inoculation into naïve recipient mice via tail vein injection.

Example 4 Generation of cps1-1 Mutant

A modified hit and run mutagenesis was devised for knocking out the T. gondii gene encoding CPSII. First, a new plasmid vector was developed for positive and negative selection analogous to the plasmid described by Fox, et al. ((1999) Mol. Biochem. Parasitol. 98:93-103), except using the herpes simplex virus type I thymidine kinase (TK) gene instead of bacterial cytosine deaminase in the linker region of DHFR-TS. To create this plasmid two primers, a forward primer GGG AGA TCT ATG GCT TCG TAC CCC GGC CAT CAA (SEQ ID NO:10) and a reverse primer GGG GAT CCT CAG TTA GCC TCC CCC ATC TCC CG (SEQ ID NO:11) were used to PCR amplify, via standard conditions, the ganciclovir hypersensitive TK75 HSVTK allele (Black, et al. (1996) Proc. Natl Acad. Sci. USA 93:3525-3529). The forward primer contains a BglII site and the reverse primer a BamHI site. Following BamHI and BglII digestion of the ˜1130 bp PCR product, the TK allele was ligated into plasmid pDHFRm2m3-FLG-TS which was digested at the unique BamHI site in the FLAG epitope linker. The TK PCR primers were designed to join with pDHFRm2m3-FLG-TS to produce an in-frame insertion of TK between DHFR and TS in a plasmid called pDHFRm2m3-TK-TS, similar to previously described pDHFRm2m3-CD-TS plasmid (Fox, et al. (1999) supra). The trifunctional enzyme plasmid with TK was tested to confirm function of all three enzymes. Transfection of T. gondii with pDHFRm2m3-TK-TS and selection in 1 μM pyrimethamine produced parasites resistant to pyrimethamine. All subclones of T. gondii transfected with pDHFRm2m3-TK-TS (more than 100 clones of T. gondii) that are pyrimethamine resistant uniformly and concomitantly become sensitive to minute concentrations of ganciclovir. All T. gondii carrying a single allele of TK (or more than one allele) from pDHFRm2m3-TK-TS do not form plaques in 0.5 μm ganciclovir.

The TgH 2-11 clone of T. gondii CPSII was fused with the pDHFRm2m3-TK-TS plasmid to create a new plasmid suitable for modified hit and run mutagenesis. First, a 0.5 Kb segment of TgH 2-11 was removed from the 3′ end by digestion with BglII and BamHI. Resulting 2.6 and 1.2 kb DNA fragments were resolved in agarose and the 2.7 kb 3′ BamHI/BglII fragment was religated with the large DNA fragment from the same digest which contained the 5′ side of the HindIII fragment and the plasmid DNA. The correct orientation was mapped by restriction digestion. This created a modified HindIII fragment of the CPSII gene (i.e., nucleotide positions 14622 and 21190 of SEQ ID NO:1) with a central 1.2 Kb BamHI fragment deleted (i.e., nucleotides 16882 to 17973 of SEQ ID NO:1), resulting in the removal of amino acids 723 and 1070 based on numbering of the T. cruzi CPSII protein (see also FIG. 1). Finally, the trifunctional DHFR-TK-TS enzyme from pDHFRm2m3-TK-TS was added into the deleted HindIII (delta 1.2 Kb BamHI) plasmid by digestion of pDHFRm2m3-TK-TS with NheI and XbaI and ligation into the unique XbaI site of the deleted 1.2 Kb BamHI HindIII TgH 2-11 plasmid. A clone with inverted directionality of DHFR-TK-TS and CPSII expression was obtained and this plasmid was called p53KOX3-1R. The targeted disruption of the endogenous T. gondii CPSII gene with p53KOX3-1R involved insertion of this plasmid by a single cross-over recombination to yield a C-terminal truncated CPSII with the 1.2 Kb BamHI deletion described above with a normal endogenous promoter but no untranslated 3′ regulatory region, and a duplicated CPSII allele with a N-terminal truncated (deleting everything before the HindII site at amino acid 663) CPSII and a normal non-translated 3′ regulatory region but no T. gondii promoter. Thus, single homologous crossover between truncated and deleted CPSII contained in p53KOX3-1R and the endogenous T. gondii locus generated 5′- and 3′-truncated cpsII alleles at the CPSII genomic locus which failed to produce a functional CPSII protein.

Wild-type RH parasites were transfected with p53KOX3-1R and selected in the presence of 1 μM pyrimethamine and 200 μM uracil. Following lysis of the primary flask with p53KOX3-1R transfected parasites after 4 days of growth in 1 μM pyrimethamine plus 200 μM uracil, parasites were diluted 1:100 and inoculated into a second flask of fresh HFF cells under the same growth conditions. The second growth cycle is necessary for efficient selection of stable plasmid integration under pyrimethamine selection. Thus, transfected parasites must undergo approximately 25 cycles of replication prior to subcloning and screening for potential mutants. It is obvious that any mutant with any moderate or significant defect in growth rate would be quickly diluted in number by rapidly growing parasite in the mixed cultures. Following the second growth cycle of the primary transfection of p53KOX3-1R in pyrimethamine plus uracil medium, parasites were subcloned into a duet of 96-well trays with or without uracil supplementation. Individual wells were scored microscopically at 4-5 days post subcloning to mark wells with one viable parasite (a subclone) based on the presence of a single zone of parasite growth in that well. Typically 10 to 20 wells of a 96-well tray were successful subclones. Successful subclone wells were individually mixed by washing up and down with a 50 μl pipette to mix parasites to infect the whole HIFF monolayer in that well. Typically 3 further days of incubation produced total well lysis and many free infectious extracellular parasites. From these wells parasites were individually picked and inoculated (separate additions) into parallel wells of HFF cells in 96-well trays that contained the same growth medium (pyrimethamine plus uracil 200 μM) or trays only containing pyrimethamine 1 μM. If the CPSII locus failed to produce a functional CPSII protein in any of the T. gondii subclones a difference in growth rate could be detected by visual microscopic examination of identically inoculated wells. No less than 1 μl was tested at this point due to reliability of transfer and parasite number of inoculum. Thus, the concentration of residual uracil in the “no uracil” tray was actually still ˜1 μM. More than 800 subclones of T. gondii were eventually screened using the above assays, with more than 200 subclones being generated in each of four independent selections and transfection experiments with p53KOX3-1R. Following an initial assessment of growth rate estimate in the plus uracil or “minus uracil” condition, a number of putative clones were evaluated in a second test of uracil growth dependence. Following a second positive test of uracil growth dependence, a third test using 25 cm² HFF flask was performed under conditions of a uracil concentration less than 0.1 μM. From these selections four T. gondii mutants were obtained which had a quantitative assessment of at least a detectable growth dependency on addition of uracil to the growth medium. These were putative T. gondii uracil auxotrophs. One independent transfection produced mutant cps1, a second independent transfection produced mutant cps2, and a third independent transfection produced mutants cps3 and cps4. Each of these mutants was found to be highly sensitive to ganciclovir, loosing ability to form plaques in only 0.5 μM ganciclovir. The mutants were grown and genomic DNA was isolated from each mutant and wild-type RH parasites from the contents of 2 or more 25 cm² flasks for each DNA isolation to document integration of targeting disruption plasmid p53KOX3-1R into the endogenous CPSII locus by homologous recombination. The plasmid p53KOX3-IR could form two general patterns of integration based on recombination either 5′ of the BamHI deletion, or recombination 3′ of the BamHI deletion. HindII digested cps1, cps2, cps3, cps4 and RH parasite genomic DNA was subjected to Southern blot analysis and hybridized to labeled gel-purified 6.6 Kb HindIII fragment of TgH 2-11 encoding T. gondii CPSII sequences. A 5′ cpsII integration would produce at least fragment sizes of ˜6.5 Kb and 7.8 Kb following digestion with HindIII, whereas a 3′ cpsII integration site would produce at least fragments of 5.0 Kb and 7.8 Kb when digested with HindIII. If the plasmid were duplicated at the time of integration, which is seen frequently with the pDHFRm2m3-TS plasmid backbone (Sullivan, et al. (1999) Mol. Biochem. Parasitol. 103:1-14), then an additional fragment at 7.8 or 9.5 Kb could be generated by integration at endogenous CPSII. Each of the selected putative cpsII mutants had undergone an integration of plasmid p53KOX3-1R at either the 5′ location (cps1, cps2, cps3, or the 3′ location, cps4). Mutant cps4 had multiple bands between 7.8 and 9.5 Kb and additional bands at higher molecular weights suggesting integration of plasmid at CPSII and other loci. In contrast, mutants cps1, cps2 and cps3, obtained in independent transfections and selection, had identical patterns of hybridization to CPSII DNA suggesting that the targeting plasmid p53KOX3-IR only integrated into the 5′ site of the CPSII target region and each mutant had duplicated the plasmid DNA upon integration (the 9.5 Kb DNA band). Thus, successful targeting to and disruption of the T. gondii CPSII locus was demonstrated.

Each of the mutants (cps1, cps2, cps3, and cps4) had a phenotype of uracil growth dependence. However, all of these mutants are somewhat “leaky” in that there was not an absolute growth (replication) dependence on uracil addition to the growth medium for replication. Each of these mutants grows at a moderate (½ of normal) growth rate for the first 2 days following infection of a host cell producing vacuoles that contained 16 to 32 parasites by 3 days post infection. In contrast, RH parasites are lysed out of their primary vacuoles at this time (3 days, >64 parasites). However, the cps mutants slow after 3 days and many parasites never (about ⅓) break out of their primary vacuole. If the primary vacuole breaks, a few parasites can be detected at the site of infection but the infection site always involves a small zone of infection that never forms a visible plaque in a standard 7 day plaque assay. To quantitate growth of these mutants, HFF flasks were inoculated with opal or cps2 parasites (about a multiplicity of infection (MOI) of 1 parasite per 20 HFF cells). After 2 hours of attachment and invasion (all of these mutants have normal attachment and invasion phenotypes as scored by counting percent entered parasites into host cells as a function of time post-inoculation), different concentrations of different pyrimidine compound was added to parallel infected HFF flasks. As a function of time post pyrimidine addition (t=0 hour) the number of parasites per vacuole was scored for 50 vacuoles as described by Fox et al. ((1999) supra). The number of parasite doublings was calculated based on 1 parasite entering each primary vacuole. Thus, 1 doubling=2 parasites per vacuole, 3 doublings=8 parasites per vacuole, and 5 doublings=32 parasites per vacuole. The pyrimidine dependence of cps1 and cps2 replication (doublings of parasites in the vacuole) was plotted graphically. Relatively low concentrations of uracil, uridine, deoxyuridine, cytidine, and deoxycytidine completely rescued the growth rate of cps1 and cps2 mutants to wild-type RH levels. This pattern of rescue is precisely consistent with the limited set of salvage enzymes available to T. gondii suggesting that these mutants have a defect in de novo pyrimidine synthesis that can be corrected by salvage of pyrimidines from exogenously supplied pyrimidines in growth medium in vitro. Cytosine, as expected, did not rescue at all. The response to thymine and thymidine was additionally informative about the cause of the growth defect in cps1 and cps2. Moderate concentrations of thymine or thymidine partially rescued the replication of cps1 and cps2, whereas very high concentrations of these pyrimidines did not rescue replication. This is believed to be caused by the putative defect in the cps1 and cps2 mutants which is a reduced “pool” size of UMP. If UMP pools (ultimately used for RNA and DNA synthesis) are lowered it follows that a resulting decrease in TMP pools is expected since UMP is the precursor of TMP in T. gondii and all other apicomplexan parasites which normally lack TK activity. However, since cps1 and cps2 now express a TK activity carried into the parasite by the p53KOX3-1R plasmid, exemplified by sensitivity of these mutants to ganciclovir (specific to HSV TK), feeding parasites either thymine or thymidine is expected to increase TMP pools. Thus, it appears that moderate levels of thymine or thymidine partially rescue growth of cps1 and cps2 by restoring TMP pools. These data indicate that there is indeed a defect in accumulation of UMP and TMP pools in the cps1 and cps2 mutants. The defect is most easily rescued by feeding the parasite pyrimidines that can be incorporated (uracil, uridine, deoxyuridine, cytidine, deoxycytidine) and can be partially rescued with thymine or thymidine (the TMP pool). Since the parasite has no mechanism to convert TMP back to UMP there is still a defect in the UMP pool in cps1 and cps2 even with added thymine or thymidine and rescue is never complete. The ability of cps1 and cps2 mutants to form plaques on HFF monolayers in the standard 7 day assay paralleled the pyrimidine dependence of parasite replication in vacuoles.

The cps1 and cps2 mutants were inoculated intraperitoneally (i.p.) into balb/c mice to measure parasite virulence compared to virulent RH parasites. Both mutant cps1 and cps2 had equal virulence as RH parasites in balb/c mice (Jackson Labs), killing all mice within 10 days of i.p. inoculation (group size was four mice per parasite strain). Only 100 parasites (˜50 PFU) of each parasite strain was needed to kill all mice in each group. This pattern of virulence can be understood by re-examining the pyrimidine dependence of cps1 and cps2 growth. Uridine is believed to be the pyrimidine responsible for virulence of cps1 and cps2 in mice. Only 5 μM uridine completely rescues normal plaque size of cps1 and cps2 in vitro. Plasma concentrations of uridine in mice are approximately 5 to 10 μM. Thus, as suggested by the “leaky” phenotype, cps1 and cps2 are not pyrimidines auxotrophs and therefore grow normally in mice and are not attenuated.

It is believed that the single recombination into the CPSII locus only partially disrupted expression of CPSII activity in cps1 and cps2. Accordingly, cps1 and cps2 mutants were not “complete” pyrimidine auxotrophs and did not lack a functional CPSII protein. Cps1 and cps2 were thus utilized as the parent strain background in which to select a more highly attenuated pyrimidine auxotroph mutant. Both the cps1 and cps2 mutants express a TK allele which was inserted into the CPSII locus. Hence, cps1 and cps2 mutants were grown for several generations in the absence of pyrimethamine and in the presence of 200 μM uracil. Then, approximately 1-2×10⁵ cps1 or cps2 parasites was inoculated into a 25 cm² HFF flask and selected negatively in the presence of 10 μM ganciclovir plus 200 μM uracil. This is 20 times the dose of ganciclovir necessary to completely block plaque formation of these mutants. After approximately 10 days, an outgrowth of viable parasites was observed for both the cps1 and cps2 selections. The parasites which were growing in ganciclovir plus uracil were subcloned in ganciclovir and uracil (same conditions) and individual clones, cps1-1 and cps2-1, were identified from each parent, respectively, for further analysis. The cps1-1 and cps2-1 subclones were first tested for their sensitivity to pyrimethamine. The theory of negative selective in only ganciclovir is that a mutant that disrupts expression of the TK allele should simultaneously acquire sensitivity to pyrimethamine due to loss of the expression of the fused trifunctional DHFR-TK-TS transgene(s) inserted into the CPSII locus of cps1 and cps2. Indeed, loss of sensitivity to ganciclovir (10 μM) in cps1-1 and cps2-2 correlated perfectly with a gain of sensitivity to pyrimethamine (1 μM) in both replication and plaque assays.

Evaluation of the pyrimidine dependence of growth of cps1-1 and cps2-1 compared to the cps1 and cps2 parents, respectively, revealed that the newly selected mutants (cps1-1 and cps2-1) were absolute pyrimidine auxotrophs and lacked a functional CPSII protein. No pyrimidine compound at less than 25 μM could rescue plaque formation of cps1-1 or cps2-1. Further, only uracil and deoxyuridine provided significant growth rescue, and only in relatively high doses. Uridine was quite poor at rescue of plaque formation in cps1-1 and cps2-1. In pyrimidine concentrations up to 200 μM only uracil completely rescued plaque formation of cps1-1 and cps2-1. In addition, as expected from ganciclovir resistance (no TK expression) phenotype, no response was detected to thymine or thymidine.

A more detailed growth response to pyrimidine, measured as parasite replications (doublings) was performed for cps1-1 and cps2-1 and compared to the results previously obtained for cps2 and cps2. In the absence of added pyrimidines or the presence of even high concentrations of thymine, thymidine, deoxycytidine, cytidine, or cytosine, a cps1-1 or cps2-1 parasite that entered a vacuole in a host HFF cell remained as a single non-replicated parasite, not only in a 36-hour replication assay, but also upon continued incubation of infected cultures in vitro. Uridine rescue was poor, with a slow restoration of growth at very high amounts, >400 μM. Deoxyuridine rescue was significant, but again, full growth rate was not restored at any concentration of deoxyuridine. Rescue with uracil was robust, but only in a limited range of concentration. Full restoration of growth rate in cps1-1 and cps2-1 was possible only with uracil added between 200 and 400 μM. Lower concentrations of uracil rescued poorly and concentrations of uracil higher than ˜500 μM reduced the growth rate of cps1-1 and cps2-1 significantly. In fact, cps1-1 and cps2-1, as well as cps1 and cps2, do not even form plaques in 2000 or 4000 μM uracil, conditions with no effect on RH parasite plaques. This result is dichotomous and suggests that there is an intimate, regulatory loop in T. gondii pyrimidine salvage that is down-regulated by very high concentrations of uracil. This phenotype can only be observed in T. gondii pyrimidine auxotroph mutants, not in wild-type parasites with intact de novo pyrimidine synthesis pathways.

The cps1-1 and cps2-1 mutants were examined for virulence in balb/c mice. An i.p. administered inoculum of 100 parasites of either cps1-1 or cps2-1 had no measured virulence in balb/c mice, compared to the same dose of RH, cps1 or cps2 which were virulent. The avirulence of cps1-1 and cps2-1 correlates well with the pyrimidine dependence of parasite growth in vitro. The high concentrations of pyrimidines needed for growth of mutant cps1-1 and cps2-1 are simply not available in mammals such as mice. Other mammals including humans and other vertebrates are also not expected to have sufficiently high pyrimidine concentration to support growth of these mutants.

Higher dose virulence studies were also performed for the mutant cps1-1. Pyrimidine auxotroph cps1-1 was completely avirulent in Balb/c mice at doses equal to or greater than 10⁷ parasites delivered by i.p. inoculation.

The ability of the pyrimidine auxotroph mutants of the present invention to protect against infections was demonstrated. After 40 days i.p. inoculation of cps1-1 in balb/c mice, the same group of 4 mice were challenged with 200 parasites of the virulent RH strain (a 100% lethal dose) Mice originally inoculated with 10⁷ or 10⁵ cps1-1 parasites were completely protected from RH challenge. In contrast, mice receiving only the lowest dose of 10³ cps1-1 parasites were completely unprotected from this RH parasite challenge. Thus, the pyrimidine auxotroph mutant given at appropriate dose is capable of protecting mice from lethal RH parasite challenge.

Safety of the pyrimidine auxotroph mutants of the present invention was also evaluated. A group of balb/c mice were inoculated with 10⁸ cps1-1 parasites and all survived at least 24 days post inoculation. A more rigorous test of safety was performed in immunocompromised mammals. Before the present invention, no T. gondii mutant had been isolated that would itself not kill gamma interferon homozygous knock-out mice (gko mice) (Jackson Labs, strain JR2286) (Radke and White (1999) Infect. Immun. 67:5292-5297). Gamma interferon homozygous knock-out mice were inoculated with various doses of the pyrimidine auxotroph cps1-1 or a lethal low dose of RH parasites. Doses of cps1-1 (i.p. administered) at 10², 10⁴ and 10⁶ did not kill any of the 4 gamma interferon homozygous knock-out mice in each group, whereas all mice receiving RH parasites died within 8 days. Mutant cps2-1, was also avirulent in homozygous gamma interferon knock-out mice. Thus, the pyrimidine auxotroph mutants of the present invention are the first described T. gondii parasite isolates that are completely attenuated even in severely immunocompromised mice. The cps1-1 and cps2-1 mutants attach and invade as efficiently as wild-type RH parasites in the absence or presence of pyrimidine in vitro in HFF cells. Thus the growth defect seen in immunocompromised mice is due to a block in intracellular replication only.

The pyrimidine growth dependence of mutant cps1-1 on the pyrimidine salvage pathway was further documented in thymidine interference experiments. Mutant cps1-1 plaques well in 250 μM uracil or deoxyuridine, but not in 1000 μM thymidine. Since thymidine at 1000 μM is known to inhibit approximately 90% of the parasite nucleoside phosphorylase activity specific for cleavage of deoxyuridine (Iltzsch, (1993) J. Euk. Micro. 40:24-28), growth of cps1-1 was tested in combinations of 1000 μM thymidine and 250 μM uracil or 250 μM deoxyuridine. All pyrimidine salvage in T. gondii must pass through uracil and UPRT conversion of uracil to UMP. In contrast, thymidine has no effect on UPRT activity. Thus, if growth of cps1-1 were dependent on deoxyuridine supplementation then replication in this condition may be inhibited by co-supplementing with 1000 μM thymidine. It was found that thymidine did block deoxyuridine-dependent growth of cps1-1 in these experiments by inhibiting nucleoside phosphorylase. However, thymidine did not affect UPRT or uracil dependent growth of cps1-1. Thus, when cps1-1 is grown in deoxyuridine the parasite strictly requires nucleoside phosphorylase activity to cleave deoxyuridine for growth. These data show cps1-1 and cps2-1 to have a marked defect in de novo pyrimidine synthesis and depleted UMP pools. Thus, cps1-1 and cps2-1 rely strictly on pyrimidine salvage enzymes for growth and further, the HFF host cell in vitro cannot supply sufficient pyrimidines for growth of cps1-1 or cps2-1. These data indicate that the pyrimidine auxotroph mutants cps1-1 and cps2-1 can be used in screening assays to identify compounds as inhibitors of various salvage enzymes when the replication of T. gondii is dependent on salvage pathways. Such potential inhibitors can only be identified using a pyrimidine auxotroph such as provided in the instant invention.

Survival, persistence, and reversion potential of pyrimidine auxotroph mutants cps1-1 and cps2-1 were also assessed. Ability of T. gondii mutants cps1-1 and cps2-1 to survive and persist intracellularly was determined in an in vitro survivability assay. From microscopic examination of cps1-1 and cps2-1, it is known that in the absence of pyrimidine addition the mutants attach and invade at normal efficiency and a single parasite can be observed in a small vacuole. With no pyrimidines added to growth medium the single parasite in the small vacuole remains as a non-replicated single parasite indefinitely. After 2 days of pyrimidine starvation, typically one bright blue (translucent) circular structure or 2 structures about 1 micron in diameter become apparent in many parasites and in most parasites by day 3 to day 4 of pyrimidine starvation. Thus, an assay was devised to measure whether the single non-replicating parasites inside the host cells were viable or non-viable. HFF flasks were inoculated with various parasite doses. At t=0 hours medium was changed, and pyrimidine starvation started. Then, at various time points (in days), cultures of pyrimidine starved cps1-1 and cps2-1 parasites were “rescued” by addition of 300 μM uracil. Incubation of rescued cultures was performed for 7 days in a plaque assay. All cultures were then examined for evidence of small micro-plaques by microscopic examination. Cultures were also stained for normal plaques and plaques were counted. The data from this assay indicates that parasites rapidly lose viability (loss of pyrimidine rescue). This loss roughly correlates with the appearance of the small bright blue circular structure in the intracellular non-replicating parasite that is starved of pyrimidines. Thus, simple culture of cps1-1 and cps2-1 in normal growth medium results in a pyrimidine starvation that efficiently kills intracellular cps1-1 and cps2-1 parasites. Thirty-two days of pyrimidine starvation was sufficient to kill at least 10⁶ PFU which was added to a single 25 cm² HFF flask. In these experiments it was also shown that addition of more than 2×10⁷ cps1-1 or cps2-1 parasites (MOI>10 parasites per HFF cell) to a single 25 cm² HFF flask resulted in all HFF cells becoming multiply infected with parasites each being a single parasite in an individual vacuole. Unexpectedly, even at these high MOI's of infection of HFF the host cell appeared perfectly normal, other than having 5 to 10 parasites within each cell on average. Thus, cps1-1 and cps2-1 also provide a useful strain of T. gondii to further analyze host-parasite interaction biology of obligate intracellular parasites. As demonstrated herein, these strains are particularly useful in further cell biological evaluation of the pyrimidine starvation phenotype or death phenotype.

To assess reversion, 10⁶ to 10⁷ cps1-1 or cps2-1 parasites were periodically inoculated into HFF flasks supplemented with 5 μM uridine. This concentration of uridine is sufficient to rescue the parent strains (cps1 and cps2) of cps1-1 and cps2-1. However, it is insufficient to support any growth of cps1-1 or cps2-1. In multiple experiments involving a total of 5×10⁸ to 1×10⁹ parasites of cps1-1 and cps2-1, no revertants were observed.

CPSII enzyme activity and thymidine kinase activity in parasite protein extracts derived from mutant or wild-type parasites were also assessed. In these experiments, parasites were grown under appropriate conditions in multiple 25 cm² flasks or in 150 cm² flasks until lysis of the host monolayer. Extracellular parasites were purified through 3 micron nucleopore filters and parasites in parasite pellets were lysed in the presence of protease inhibitors to generate protein extracts for enzyme assays. The CPSII and TK enzyme activities in the various parasite extracts was determined in enzyme assays. The enzyme activity data indicates that the cps1 and cps2 mutants are partial knock-outs of CPSII activity compared to the activity measured in the wild-type RH strain. Furthermore, as the bulk of data from pyrimidine rescue experiments indicated, no CPSII activity was detected in pyrimidine auxotroph mutants cps1-1 and cps2-1. Measurement of TK activity corresponded with previously determined sensitivity to ganciclovir. Parasites that were sensitive to ganciclovir had TK activity, whereas parasites that became resistant to ganciclovir lost TK activity. These measurements of CPSII enzyme activity confirm that cps1-1 and cps2-1 mutants lack a functional cpsII enzyme which results in blocking de novo synthesis of pyrimidines.

Example 5 T. gondii Vaccine to Elicit Cell-Mediated Immunity

This example describes the host immune response to cps1-1 using various routes of immunization, as well as APC cell-dependence of immune control of infection, the type of immunity responsible for long lasting protection and, the dynamics of cell and cytokine profiles during the first eight days of exposure to cps1-1.

Cps1-1 Tachyzoites Induce a Completely Protective Long Lasting Immune Response Against High Dose Lethal Challenge with Hypervirulent RH Tachyzoites. Having demonstrated herein that live attenuated cps1-1 tachyzoites protect Type II T gondii resistant BALB/C mice against a low dose lethal challenge in a single inoculation dose, immunization with live attenuated cps1-1 and mechanisms of immune protection elicited in the priming phase of the highly sensitive C57BL/6 mouse background was analyzed. Live attenuated cps1-1 tachyzoites were effective in producing immunological protection against high dose lethal challenge in C57Bl/6 mice. C57BL/6 mice were immunized intraperitoneally twice with 1×10⁶ cps1-1 tachyzoites, 14 days apart, then challenged i.p. 4 weeks after the final immunization with a high 1×10³ lethal dose of RH tachyzoites. Mice immunized with the cps1-1 vaccine were completely protected (100%) when followed up to 30 days post-challenge, whereas naïve mice uniformly succumbed to infection by day 10 post-challenge. Cps1-1 immunized mice were continuously monitored over 18 months post-challenge and uniformly survived challenge infection to old age, indicting that the cps1-1 vaccine induces long lasting protective immunity.

Intravenous Immunization with cps1-1 Tachyzoites Alone or ex vivo cps1-1 Infected DCs or PECs Induce Long Lasting Protective Immunity. Numerous studies have reported that the route of immunization is a critical factor in determining vaccine effectiveness (Bourquin, et al. (1998) Infect. Immun. 66:4867-4874; McLeod, et al. (1988) J. Immunol. 140:1632-1637; Aline, et al. (2004) Infect. Immun. 72:4127-4137). To explore the importance of the route of vaccination to the development of long lasting immunity in the cps1-1 model, C57Bl/6 mice were immunized either once i.p., or subcutaneously (s.c.), or twice i.p, or s.c. with 1×10⁶ cps1-1 tachyzoites. Four weeks post-immunization, mice were challenged i.p. with a high 5×10⁴ dose of high in vitro passaged hypervirulent Type II strain PLK tachyzoites and percent survival was monitored to 30 days post-challenge (Howe, et al. (1996) Infect. Immun. 64:5193-5198; Sibley & Howe (1996) Curr. Top. Microbiol. Immunol. 219:3-15). Mice immunized twice i.p. were completely protected against this high dose PLK challenge while mice immunized once i.p. showed lower survival (FIG. 2). Unexpectedly, mice immunized s.c. did not survive challenge infection (FIG. 2). The previously observed protection conferred by immunization with temperature-sensitive strain ts-4 tachyzoites (McLeod, et al. (1988) supra) must require parasite replication which does not occur in the non-replicating cps1-1 vaccine.

To further elucidate effective routes of cps1-1 immunization, it was determined whether immunizations given intravenously (i.v.) were capable of inducing protective immunity against lethal RH challenge. Mice were immunized once with 1×10⁶ cps1-1 tachyzoites i.v. then challenged 2 months (FIG. 2A) or 6 months (FIG. 2B) post-immunization with a low dose (1×10⁶) of RH tachyzoites i.p. and survival was monitored. All mice immunized i.v. with cps1-1 survived challenge whereas all naïve control mice given PBS alone succumbed to challenge infection. Unexpectedly, naïve mice infected i.v. with RH tachyzoites succumbed 2 days earlier than naïve mice injected i.p., indicating that parasitemia is critical to lethal pathogenesis. Since the i.v. route was effective in inducing long lasting protective immunity in a single inoculation dose and previous studies exploring the use of professional APCs loaded ex vivo with antigen have achieved protection against lethal challenge, the effectiveness of specific host APC types in the development of protective immunity was examined (Bourguin, et al. (1998) Infect. Immun. 66:4867-4874; Aline, et al. (2004) Infect. Immun. 72:4127-4137). Mice were immunized once with ex vivo cps1-1 infected DCs, PECs and macrophages derived from resident PECs i.v. and then were challenged at 2 (FIG. 2A) or 6 months (FIG. 2B) post-immunization with a low dose of RH tachyzoites and survival was monitored. Immunization with ex vivo cps1-1 infected DCs, PECS, or PEC-derived macrophages resulted in nearly complete survival of RH challenged mice at 2 months post-immunization and was not significantly different from mice immunized with cps1-1 i.v. (FIG. 2A). When lethal challenge was administered at 6 months (FIG. 2C), differences in percent survival of mice immunized with ex vivo cps1-1 infected DCs (83% survival), PECs (50% survival), and PEC-derived macrophages (33% survival) were observed and all cps1-1 immunized mice survived longer than PBS naïve control mice (all p-values=0.0009). Percent survival of mice immunized with ex vivo cps1-1 infected DCs and PECs was of similar statistical significance to mice immunized i.v. with cps1-1 tachyzoites, indicating that although peritoneal macrophages were effective at inducing protection at 2 months, this protection was not as long lasting as that induced by ex vivo cps1-1 infected DCs and PECs (compare FIGS. 2B and 2C)

The Potent Long Lasting Protective Immune Response Elicited by Immunization with Live Attenuated cps1-1 Vaccine is Primarily CD8⁺ T Cell-Mediated and Can Re Adoptively Transferred. The paradigm of a Th-1 inflammatory response inducing cell-mediated immunity providing long term protection involving both CD8+ and CD4+ T cells for resistance to active T. gondii infection is well-established (Suzuki and Remington (1988) J. Immunol. 140:3943-3946; Gazzinelli, et al. (1991) J. Immunol. 146:286-292). Although during the innate response NK cells may play a role in controlling the initial parasite infection, the primary effector cell population responsible for the adaptive cell-mediated protective response is CD8⁺ T cells (Subauste, et al. (1991) J. Immunol. 147:3955-3959; Hakim, et al. (1991) J. Immunol. 147:2310-2316). To determine whether CD8⁺ T cells are the primary effector cells responsible for the adaptive immune response after i.p. immunization with cps1-1, antibody depletion of specific T cell populations from immunized mice was used and survival of T cell-depleted mice was measured against lethal challenge. As a control for determining if T cells are the primary effector arm of adaptive immunity responsible for long lasting protection in the present vaccine model, B cell-deficient (μMT) mice were also immunized to assess whether B cells are required for immune responses leading to protective immunity. Wild-type C57Bl/6 and μMT mice were immunized i.p. with cps1-1. C57Bl/6 mice were then antibody depleted of either CD8⁺ T cells, CD4⁺ T cells, or both CD4⁺ and CD8⁺ T cells whereas μMT mice were not treated. All immunized mice were high dose challenged with 1×10³ RH tachyzoites i.p. and monitored for survival. It was observed that 100% of CD4⁺ T cell-depleted mice survived whereas only 25% of either CD8⁺- or CD8⁺/CD4⁺-depleted mice survived the challenge infection (FIG. 3A). Although cps1-1 immunized μMT mice survived longer than non-immunized naive μMT mice, all immunized μMT mice succumbed to infection by day 27 post-challenge (FIG. 3B). This result indicates that B cells may adopt a subordinate role as effector cells via antibody production, or that during host response to immunization a deficiency occurs in the development of an effective memory CD8⁺ T cell population leading to a less potent protective response (Sayles, et al. (2000) Infect. Immun. 68:1026-1033; Langhorne, et al. (1998) Proc Natl. Acad. Sci. USA 95:1730-1734; Matter, et al. (2005) Eur. J. Immunol. 35:3229-3239). These results demonstrate that CD8⁺ T cells are the main effector cell involved in the protective immunity induced by immunization with cps1-1.

Adoptive transfer was also carried out to confirm that CD8⁺ T cells were the primary effector mechanism against T gondii infection that develop in response to immunization with cps1-1. Splenocytes were harvested 30 days after cps1-1 immunization and purified CD8⁺ T cells, B cells, or whole splenocytes were adoptively transferred into naïve recipients. Recipient naïve mice were then challenged with 1×10³ RH tachyzoites and percent survival was monitored. Mice receiving 1×10⁷ whole splenocyte-derived cells or 5×10⁶ B cells succumbed to infection by day 10 post-challenge. In contrast, all naïve mice receiving 1×10⁷ purified CD8⁺ T cells or 4×10⁷ whole spleen cells survived challenge infection (FIG. 3C). These results confirm that T. gondii-pecific CD8⁺ T cells induced by immunization with cps1-1 are the primary effector cells required for adaptive immunity and long lasting protective immunity.

IgG2a is Present in Serum from cps1-1 Immunized Mice Indicative of a Th-1 Immune Response. Specific antibody subclasses are one indicator of the type of T helper cell response induced by infection. A T helper type I cell-biased population induces the production of the immunoglobulin subclass IgG2a (Snapper & Paul (1987) Science 236:944-947; Sornasse, et al. (1992) J. Exp. Med. 175:15-21). Infection with virulent T gondii parasites, as well as immunization with an attenuated temperature sensitive mutant (ts-4) or DCs pulsed with T. gondii antigens, result in IgG positive serum titers that predominantly include the IgG2a subclass (Bourguin, et al. (1998) supra; Waldeland, et al. (1983) J. Parasitol. 69:171-175; Johnson & Sayles (2002) Infect. Immun. 70:185-191). C57Bl/6 mice were immunized with cps1-1 and sera were collected four weeks after the final immunization and examined for titers of whole IgG, IgG1 and IgG2a. Anti-toxoplasma serum titers of total IgG and subclasses IgG1 and IgG2a were nearly equivalent (Table 1). These results for serum IgG titers were similar to those previously reported in response to immunization with ts-4 (Waldeland, et al. (1983) supra). The presence of significant levels of IgG2a in sera from cps1-1 immunized mice indicates the induction of a Th-1-biased T helper cell response.

TABLE 1 IgG Mean Titer SEM Mean A450 SEM IgG H + L 14103.8 781.8 0.546 0.011 IgG1 12020.3 2921.9 0.095 0.009 IgG2a 13616.1 1126.3 0.398 0.027 Titers were calculated via dilution at which samples from immunized mice were equivalent to unimmunized control sera. Absorbance at 450 nm was recorded for 1:100 dilutions. All experiments were performed with n = 4 mice per group. The data are representative of two experiments with similar results and indicate the mean ± SEM.

Inflammatory Cell Infiltrate Response to cps1-1, in the Absence of Replication and Growth Associated Host Tissue Destruction is Faster and Less Potent Than Response to RH Infection. During T gondii infection, inflammatory cells infiltrate into the site of infection, indicate the type and magnitude of an immune response, and potentially relevant mechanisms in directly or indirectly controlling the infection (Bennouna, et al. (2003) J. Immunol. 171:6052-6058; Mordue & Sibley (2003) J. Leukoc. Biol. 74:1015-1025; Kelly, et al. (2005) Infect. Immun. 73:617-621; Robben, et al. (2005) J. Exp. Med. 201:1761-1769; Scharton-Kersten, et al. (1996) J. Immunol. 157:4045-4054). Knowledge regarding inflammation, such as cellular infiltrates in response to T gondii infection, has been elucidated with replication-competent strains that induce extensive growth-associated host tissue destruction (Scharton-Kersten, et al. (1996) Exp. Parasitol. 84:102-114; Gavrilescu, et al. (2001) J. Immunol. 167:902-909). Therefore, the magnitude and kinetics of inflammatory cell infiltrates was examined in the absence of parasite replication-associated host tissue destruction after immunization with cps1-1. For comparison, the inflammatory cell infiltrate response to RH infection, which causes significant levels of replication associated host tissue destruction, was measured. C57BL/6 mice were inoculated i.p. with either 1×10⁶ cps1-1 or 1×10³ RH tachyzoites intraperitoneally, and total PECs were isolated on days 0, 2, 4, 6, and 8 post-inoculation as described herein and enumerated (FIG. 4A). A significant (p=0.005) increase in cell numbers occurred by Day 2 post-cps1-1 inoculation followed by a steady and 3-fold significant increase (p=0.0001) by Day 6 and Day 8 as compared to Day 0 naïve controls (FIG. 4A). In contrast, RH infection induced an unexpected significant decrease (p=0.024) in total PEC numbers on Day 2 post-infection (FIG. 4A). This decrease in cell number was quickly resolved by Day 4 and the highest cell numbers were seen by Day 6 then declined by Day 8 post-infection, most likely due to significant necrosis and tissue destruction. The overall magnitude of inflammatory cell infiltrate into the site of infection was greater during RH infection than with cps1-1 immunization. While the level of cellular infiltrate was significantly greater at Day 2 post-infection with cps1-1 (p=0.003) than with RH infection, this was reversed by Day 6 post-infection, where RH PECs were 1.5-fold greater (p=0.002) than seen in cps1-1-treated mice. Total cell numbers of PECs measured at Day 8 were not significantly different between RH and cps1-1. These results reveal that i.p. inflammatory cellular infiltrate response to cps1-1 was earlier than with RH, indicating an inactivation or inhibition of the early innate immune response induced by the rapidly replicating RH parasite allowing it to gain a foothold and contribute to its lethal virulence. Moreover, the overall level of PEC infiltrate in response to cps1-1 inoculation was significantly lower than RH infection, indicating that replication-associated host tissue destruction contributed to the magnitude of the inflammatory response.

Recruitment of Specific Inflammatory Cells into the Site of Infection Occurs Earlier and is Less Potent in the Absence of Rapid Replication and Growth-Associated Host Tissue Destruction. As inflammatory cell types such as granulocytes (PMNs), macrophages, and B and T lymphocytes are important for the development of protective immunity and for direct control of primary T. gondii infection, it was of significant interest to investigate the absolute numbers and percent composition of the specific cell types contained in PECs infiltrating in response to the protective cps1-1 vaccine compared to virulent (RH) infection to examine which cell populations contribute to the control and development of long term protective immunity in the absence of rapid replication and growth-associated host tissue destruction (Mordue & Sibley (2003) J. Leukoc. Biol. 74:1015-1025; Scharton-Kersten, et al. (1996) J. Immunol. 157:4045-4054; Bliss, et al. (2000) J. Immunol. 165:4515-4521). Initial flow cytometric analysis of total PECs identified three general cell populations when using forward scatter (FSC) and side scatter (SSC) analyses. FSC^(low) SSC^(low) were classified as lymphocytes (gate R1), FSC^(high) SSC^(low-mid) were classified as macrophages/monocytes (gate R2), and FSC^(mid) SSC^(high) were classified as granulocytes/neutrophils (gate R3) (Bliss, et al. (2000) J. Immunol. 165:4515-4521; Schleicher, et al. (2005) Blood 105:1319-1328; Henderson, et al. (2003) Blood 102:328-335). Based on these criteria three individual gates were drawn to isolate each region for analysis and whose sum of data were verified and found to be equivalent to results of total events.

Granulocytes. Granulocytes are rapid responders and quickly infiltrate after T. gondii infection. These cells are required for early control of infection and are an early source of IL-12, which may set the stage for a Th-1-skewed response and early IFN-γ production (Bliss, et al. (2000) J. Immunol. 165:4515-4521; Khan, et al. (2001) J. Immunol. 166:1930-1937; Bliss, et al. (2001) Infect. Immun. 69:4898-4905; Del Rio, at al. (2001) J. Immunol. 167:6503-6509). To carry out this analysis, a double stain of Gr-1 and CD68 was used and the percent of total events in the granulocyte (R3) region were measured. The results from R3 were confirmed by back-gating from all three gates for Gr-1⁺ CD68⁻ cells and by staining for Gr-1 alone. From this analysis it was determined that R3 contained 95% of the granulocytes detected (Bliss, et al. (2000) J. Immunol. 165:4515-4521). After cps1-1 vaccination i.p. a significant 9-fold increase in the absolute number of granulocytes (p=0.026) was observed by Day 2 post-inoculation. After Day 2 the total numbers of granulocytes returned to uninfected control levels (FIG. 4B, upper panel). When the percent of Gr-1⁺ CD68⁻ cells in total events was measured, the identical pattern of granulocyte infiltration was observed with a significant increase by Day 2 post-infection (p=0.033) from 1.8% to 11.5% followed by a decrease to 1.6% by Day 4 (FIG. 4B, lower panel). In contrast to cps1-1, the absolute numbers of granulocytes responding to RH infection did not significantly increase until Day 4 where a 6.8-fold increase (p=0.001) over Day 0 was measured. This increase in granulocyte numbers continued through Day 6 with a 6.0-fold increase over Day 4 (p=0.001) followed by a significant reduction at Day 8 (p=0.001) (FIG. 4B, upper panel). This pattern was also observed in terms of the percent of total infiltrating cells being granulocytes, wherein at Day 2 post-infection 5.2% of the total cells were granulocytes, at Day 6 post-infection 15.3% of the total cells were granulocytes, and at Day 8 a 4-fold decrease was observed (FIG. 4C, lower panel). Recruitment of granulocytes after cps1-1 vaccination occurred more rapidly (peak by Day 2) as compared to RH infection (peak by Day 6) as indicated by both absolute numbers and percentages of total events (FIG. 4B). After Day 2, RH infection induced significantly greater granulocyte infiltration than did cps1-1 vaccination and these differences may have been due to a difference in initial antigen load, a delay in granulocyte infiltration by virulent RH infection, or granulocyte infiltration in response to RH infection associated host tissue destruction.

Macrophages. The level and kinetics of macrophage infiltration after infection with RH or vaccination with cps1-1 was also determined. Macrophages encompass the greatest percentage of resident PECs (˜50%) in the uninfected steady state (FIG. 4C). There is evidence that macrophages are preferentially targeted for invasion by T. gondii. Either Gr-1⁺ or Gr-1⁻ macrophages respond to infection, provide a host cell environment amenable to parasite growth and replication, and provide a first line of host defense (Mordue & Sibley (2003) J. Leukoc. Biol. 74:1015-1025; Robben, et al. (2005) J. Exp. Med. 201:17611769). To measure the absolute number and percent of macrophages in total PECs, an intracellular stain for CD68 or macrosialin was employed. In addition to CD68, PECs were also stained with the surface marker Gr-1 to detect the T. gondii-specific double-positive staining Gr-1⁺ CD68⁺ inflammatory macrophage population. The absolute numbers of CD68⁺ macrophages infiltrating into the peritoneum with cps1-1 vaccination significantly increased 1.5-fold (p<0.001) by Day 4 post-inoculation compared to Day 0 (FIG. 4C, upper panel). The maximum number of CD68⁺ cells had infiltrated by Day 6 with a 2.1-fold increase (p=0.001) in absolute numbers over Day 0 controls. This population remained present at Day 8 post-infection with no significant change. When this pattern of influx was analyzed as a percent of total PECs, it was observed that the percentage of CD68⁺ cells did not significantly change until Day 6 and Day 8, where unexpectedly CD68⁺ cell numbers decreased (p=0.011) compared to Day 0 (FIG. 4C, lower panel). Although absolute numbers of CD68⁺ macrophages increased with time in both RH infection and cps1-1 immunization, other cell populations were infiltrating at a more rapid rate by Day 6 post-inoculation.

CD68⁺ Gr-1⁺ Macrophages. Analysis was also carried out to determine the absolute number and percentage of the total PECs that were Gr-1⁺ CD68⁺ inflammatory macrophages infiltrating into the site of inoculation. At Day 0, or in uninfected naïve controls, less than 1% of the CD68⁺ cells were also Gr-1⁺ (FIG. 4D). When mice were immunized i.p. with cps1-1, a significant 168-fold increase (p<0.001) in the absolute numbers of Gr-1⁺ CD68⁺ cells was observed by Day 2 post-infection (FIG. 4D, upper panel). A significant 2.5-fold decrease (p<0.001) in the numbers of Gr-1⁺ CD68⁺ macrophages at Day 4 post-inoculation was then observed. It was surprising to also observe a second population or wave of Gr-1⁺ CD68⁺ macrophages infiltrating into the site of cps1-1 inoculation at Day 6, wherein a significant 2.5-fold increase (p=0.002) in absolute cell number (compared to Day 4 and Day 6) was observed, which was then followed by a significant 3.8-fold decrease (p=0.004) in the number of inflammatory macrophages by Day 8 post-vaccination (compared to Day 6 and Day 8). This analysis of the percent of total PECs identified as Gr-1⁺ CD68⁺ inflammatory macrophages over the course of inoculation with cps1-1 followed the pattern observed for absolute numbers (FIG. 4D, lower panel). In contrast, the percentage of CD68⁺ macrophages decreased between Day 0 and Day 8 (FIG. 4C, lower panel). These observations indicate that of the CD68⁺ resident macrophages that are present in the peritoneum at Day 0, >99% have been replaced by the T. gondii-specific inflammatory Gr-1⁺ CD68⁺ macrophages by Day 2 post-inoculation. Not wishing to be bound by theory, this is most likely due to infiltration of new Gr-1⁺ CD68⁺ inflammatory macrophages because in vitro infection of peritoneal-derived macrophages with cps1-1 under replicating or non-replicating conditions in uracil does not result in the expression of Gr-1. By Day 4 post-inoculation many of these Gr-1⁺ CD68⁺ inflammatory macrophages are cleared down to 27.5% of all CD68⁺ macrophages. By Day 6 post-inoculation, 68% of the CD68⁺ macrophages are Gr-1⁺ CD68⁺ followed by a decrease to 14.8% of all CD68⁺ cells by Day 8 post-inoculation (Mordue & Sibley (2003) J. Leukoc. Biol. 74:1015-1025; Robben, et al. (2005) J. Exp. Med. 201:17611769).

In contrast to non-replicating cps1-1 vaccine, the total CD68⁺ macrophages decreased 4-fold (p<0.001) by Day 2 post-active RH infection when compared to Day 0 (FIG. 4C, upper panel). The number of CD68⁺ macrophages then increase significantly 7-fold (p<0.001) by Day 4 post-infection compared to Day 2. By Day 6 post-infection with RH, CD68⁺ cells had increased 2-fold (p=0.032) over Day 4 and reached their highest absolute numbers at this time. By Day 8 post-infection absolute numbers of CD68⁺ macrophages remained elevated but are not significantly lower (p=0.093) than Day 6 post-infection. A similar pattern of CD68⁺ macrophages as a percent of total PECs after RH or cps1-1 inoculation was observed, indicating that CD68⁺ macrophages were recruited at the same rate regardless of replicating (RH) or non-replicating (cps1-1) parasites. Other cells types were being recruited more rapidly to the site of parasite inoculation.

When the PECs were analyzed for the absolute numbers of T. gondii-specific Gr-1⁺ CD68⁺ inflammatory macrophages during RH infection, a delay in the recruitment of these cells to the site of RH infection was observed, i.e., cells were only observed at Day 4. By Day 6, the Gr-1⁺ CD68⁺ cells had significantly increased 2-fold (p=0.041) over Day 4. The absolute numbers of Gr-1⁺ CD68⁺ cells did not significantly change between Day 6 and Day 8. The analysis of the Gr-1⁺ CD68⁺ percent composition of the total events in response to RH infection revealed a similar, but delayed pattern to that observed for cps1-1 vaccination. Upon further analysis, a similar ratio of macrophage cell types infiltrating during infection with RH was not observed as compared to cps1-1 vaccination. By Day 4 and through Day 8 post-RH infection, Gr-1⁺ CD68⁺ macrophages made up greater than 99% of the total CD68⁺ cells observed to infiltrate during the course of infection, indicating that although there may have been a similar pattern of Gr-1⁺ CD68⁺ entering and leaving the site of infection, there was a lack of balance between normal CD68⁺ activated or non-activated resident peritoneal macrophages with the activated inflammatory T. gondii-specific Gr-1⁺ CD68⁺ macrophages. This imbalance could have been in part due to the inflammatory contribution caused by virulent RH infection associated host cell and tissue destruction.

B cells. B lymphocytes play a role in mediating the outcome of infection with T. gondii because cps1-1 vaccinated μMT mice showed a delayed time to death phenotype, but uniformly did not survive a high lethal dose RH challenge (FIG. 3B). Whether this role was significant for anti-toxoplasma antibody production or enhancing memory CD8⁺ T cell responses remains under investigation (Sayles, et al. (2000) Infect. Immun. 68:1026-1033; Langhorne, et al. (1998) Proc. Natl. Acad. Sci. USA 95:1730-1734; Johnson, et al. (2002) Infect. Immun. 70:185-191). Accordingly, cell-specific differences in the host inflammatory cell infiltrate response to cps1-1 vaccination or RH infection was analyzed by measuring the absolute numbers and percentage of total PECs that were CD19⁺ B cells. Unexpectedly, 2 days after cps1-1 inoculation a significant decrease (p=0.022) in the absolute numbers (30% decrease) from naïve controls was observed (FIG. 4E, upper panel). This was followed by a significant increase in CD19⁺ B cell numbers at Day 4 (1.8-fold) and Day 6 (2-fold) with p-values equal to 0.001 and 0.003, respectively. The absolute numbers of CD19⁺ B cells then remained steady through Day 8 post-inoculation. When the percent of CD19⁺ B cells contained in total PECs after cps1-1 vaccination was assessed, 32.5% of the total PECS were CD19⁺ B cells at Day 0 (FIG. 4E, lower panel). The percentage then significantly decreased 1.9-fold to 17.2% of the total PECs, and did not significantly change through Day 8. Analysis of PECs after RH infection revealed that while the absolute numbers of CD19⁺ B cells followed a similar pattern as with cps1-1 inoculation only until Day 4, RH infection induced a significant 2-fold decrease (p=0.012) in CD19⁺ B cells by Day 6. This clearance of the CD19⁺ B cells markedly accelerated through Day 8 post-RH infection. When the percent of CD19⁺ B cells in the total number of events was measured, it was observed that the percentage of B cells in PECs in response to RH infection significantly increased (p=0.018) by Day 2 post-infection from 32.5% to 42.9% of the total (FIG. 4E, lower panel). However, by Day 4 post-infection, the percentage of B cells had significantly decreased 2.2-fold (p<0.001) below Day 0 naïve controls. By Day 6 and Day 8 post-RH infection, the percent of CD19⁺ B cells was reduced 14-fold and 35-fold, respectively, compared to Day 0 (FIG. 4E, lower panel). The retention or continual recruitment of CD19⁺ B cells during cps1-1 vaccination could be an important component in the development of CD8⁺ T cell memory through enhanced co-stimulation, antigen presentation, or by acting in an innate function if the CD19⁺ B cells were proportionally higher for B1 B cells over B2 B cells.

T cells. Resolution of infection by T. gondii requires a potent CD8⁺ T cell response. A synergistic role of both CD4⁺ and CD8⁺ T cells is required to develop this cell mediated protection (Snzuki & Remington (1988) J. Immunol. 140:3943-3946; Gazzinelli, et al. (1991) J. Immunol. 146:286-292; Subauste, et al. (1991) J. Immunol. 147:3955-3959). Due to the key role that both CD4⁺ and CD8⁺ T cells play in the outcome of T. gondii infection and development of protective immunity, the kinetics of cell infiltration to the site of inoculation was analyzed by measuring the absolute number and percent of the total PECs that were T cells (CD3⁺) and how many of those T cells were CD3⁺ CD4⁺ or CD3⁺ CD8⁺ over the course of RH infection compared to cps1-1 vaccination. This analysis indicated that, when measuring CD3 alone, the absolute number of T lymphocytes infiltrating to the site of cps1-1 inoculation significantly increased 1.9-fold (P=0.022) by Day 2 post-infection (FIG. 4F, upper panel). The increase in absolute numbers of CD3⁺ T cells continued to Day 4 and Day 6 post-infection with 4- and 11-fold increases, respectively, over Day 0. CD3⁺ T cells were at their highest number at Day 8 post-cps1-1 inoculation. The same pattern was observed when measuring the percent of CD3⁺ T cells in total PECs, wherein at Day 0 that there were less than 10% T lymphocytes, but by Day 4 post-infection this amount increased to 18.7% and at Day 6 and Day 8 the percent increased to 33.7% and 34.3%, respectively, of total PECs (FIG. 4F, lower panel). In contrast to cps1-1 vaccination, RH infection delayed an increase in total CD3⁺ T cells until Day 4 post-infection, a 1.8-fold increase was observed (FIG. 4F, upper panel). The highest numbers of CD3⁺ T cells were present by Day 6 which was then followed by a decrease in number at Day 8. The absolute numbers of T cells infiltrating into the site of infection were significantly lower (P=0.004, p<0.001, p<0.001, p<0.001) in response to RH infection than with cps1-1 vaccination for each time point beginning at Day 2 post-inoculation. When measuring the percent of CD3⁺ T cells of total PECs, it was observed that only at Day 2 was there a significant increase (P=0.035) in the percentage of total T cells after RH infection (FIG. 4F, lower panel). Soon after this peak at Day 2 post-RH infection, the percentage of CD3⁺ T cells decreased back to Day 0 levels. Overall the recruitment of CD3⁺ T cells by the host in response to cps1-1 inoculation was very robust and the presence of significant RH replication and growth-associated host tissue destruction may severely restrict CD3⁺ T cell recruitment.

As both CD4⁺ and CD8⁺ T cells play a role in controlling T. gondii infection, the kinetics and numbers of either CD4⁺ or CD8⁺ T cells infiltrating into the site of inoculation was analyzed. The CD3⁺ T cell analysis was extended to include cells that would stain double-positive for either CD3⁺ CD4⁺ or CD3⁺ CD8⁺. It was observed that after cps1-1 vaccination, both CD4⁺ and CD8⁺ T cell numbers increased significantly (p=0.029 and 0.007, respectively) 1.7- and 2.8-fold, respectively, by Day 2 post-infection (FIG. 4G, upper panel). Both CD4⁺ and CD8⁺ T cell numbers increased continuously over time and reached maximal numbers by Day 8 post-infection with absolute numbers of CD4⁺ T cells significantly greater than CD8⁺ T cells from Day 6 through Day 8 (p<0.001). However, when the different T cell populations were analyze as percentage of total PECs, the percent of CD3⁺ CD8⁺ T cells was found to increase significantly (p=0.03) early by Day 2 post-inoculation (FIG. 4G, lower panel). The CD3⁺ CD4⁺ T cell population did not significantly increase in percentage until Day 4 as compared to Day 0 (p=0.01), indicating that CD8⁺ T cells infiltrated earlier in response to cps1-1 vaccination than the CD4⁺ T cells. In contrast to vaccination with cps1-1, the absolute numbers of either CD4⁺ or CD8⁺ T cells in response to RH infection did not significantly increase until Day 4 post-infection with CD4⁺ T cells undergoing 1.4-fold and CD8⁺ T cells undergoing 2.3-fold increases (FIG. 4H, upper panel). Maximal increases in absolute numbers of both CD4⁺ and CD8⁺ T cells occurred by Day 6 post-RH infection followed by an acute and significant reduction (p=0.001 CD4; p=0.001 CD8) by Day 8 post-infection. There was no significant differences in absolute numbers between either T cell type, unlike the response to cps1-1 vaccination (compare FIGS. 4G and 4H). Percent of total event analysis revealed that only CD4⁺ T cells significantly increased at Day 2 post-RH infection (p=0.01), while CD8⁺ T cells did not significantly change as a percentage of total PECs for the entire course of infection (FIG. 4H, lower panel). The initial increase in the percent of CD4⁺ T cells in the total PEC population was reversed by Day 4 with a significant decrease (p=0.001) to below Day 0 levels. These results indicate that CD8⁺ T cells respond more rapidly than CD4⁺ T cells to vaccination with cps1-1. However, CD4⁺ T cells eventually infiltrate in greater numbers (and percentages) over CD8⁺ T cells by Day 6 and Day 8 after cps1-1 vaccination. Despite a robustly enhanced level of cellular infiltration by Day 6 in the context of RH replication and growth-associated host tissue destruction (FIG. 4A), both CD4⁺ and CD8⁺ T lymphocytes were significantly impaired in their ability to infiltrate into the site of RH infection (FIG. 4H).

The Attenuated Type I cps1-1 Vaccine Induces Early Systemic Production of IFN-γ, IL-12p40 and IL-12p70. Previous studies establish that infections with viable and replicating T. gondii parasites induce potent Th-1-biased inflammatory responses highlighted by high-level production of IFN-γ and variable levels of IL-12p70 production depending on parasite genotype (Type I, II, II) (Scharton-Kersten, et al. (1996) Exp. Parasitol. 84:102-114; Robben, et al. (2004) J. Immunol. 172:3686-3694; Scharton-Kersten, et al. (1996) J. Immunol. 157:4045-4054). In contrast, studies utilizing replicating parasites are confounded by extensive infection-associated host tissue destruction from parasite replication and growth and the resulting tissue destruction may enhance the overproduction of potentially lethal inflammatory cytokines (Gavrilescu & Denkers (2001) J. Immunol. 167:902-909). Accordingly, systemic levels of pro-inflammatory Th-1 cytokines were measured in sera at Day 0, 2, 4, 6, and 8 post-inoculation of cps1-1-vaccinated mice. In this regard, cytokine produced solely in response to this attenuated Type I parasite could be measured in the absence of growth and replication-associated host tissue destruction and compared to C57Bl/6 mice that were infected with the virulent parental Type I strain RH. The production of IL-12p40, IL-12p70, and IFN-γ was measured by ELISA.

Systemic IFN-γ and IL-12p40 in serum of mice infected with RH did not significantly increase until Day 4 post-infection and quickly rose to maximum levels by Day 6 and 8. Systemic IFN-γ at Day 0 decreased by Day 2 compared to naïve Day 0 controls (FIG. 5A). Type I RH infection induced exceedingly low levels of systemic IL-12p70 only detectable on Day 2 and Day 4 (FIG. 5B) consistent with previous reports that IL-12p70 production may only be induced by Type II parasite strains (Robben, et al. (2004) J. Immunol. 172:3686-3694). Poor IL-12p70 induction in sera of mice was proposed to be one of the reasons Type I parasite infections are universally lethal (Robben, et al. (2004) supra).

Kinetics of production of systemic IFN-γ, IL-12p40, and IL-12p70 after vaccination with the Type I cps1-1 vaccine strain derived from parental RH was completely opposite to that observed with RH infection (FIG. 5). Systemic levels of IFN-γ from mice after cps1-1 vaccination were early (p=0.03) at Day 2 post-inoculation (FIG. 5A). This systemic IFN-γ production was transient and was significantly decreased (p=0.03) by Day 4 (<10 pg/ml IFN-γ) and remained below detectable levels for the remainder of the 8-day kinetic evaluation.

In contrast to RH infection, induction of systemic IL-12p40 after cps1-1 vaccination was low with a significant systemic increase (p=0.001) detected at Day 4 (4-fold increase over Day 0 naïve mice). Levels of IL-12p40 held steady through Day 6 and Day 8 (FIG. 5B). The level of early systemic IL-12p40 production was significantly lower in response to RH infection than cps1-1 vaccination at Day 2 and Day 4 post-infection with p-values equal to 0.0001 and 0.04, respectively.

Significant systemic IL-12p70 production was induced after vaccination with the Type I attenuated cps1-1 (FIG. 5C). Cps1-1 induced systemic IL-12p70 rapidly by Day 2 post-inoculation over undetectable levels in Day 0 naïve mice. IL-12p70 levels then showed a significant increase (p=0.036) by Day 4 with a further increase by Day 6 post-inoculation and a significant decrease by Day 8 (p=0.015). When comparing the IL-12p70 production in response to Type I-matched strains, cps1-1 compared to RH, significantly greater amounts (p=0.0001) were observed for the time points of Day 4, Day 6, and Day 8 collected from mice infected with cps1-1. This was a remarkable finding because the cps1-1 parasite is an attenuated Type I parasite and production of IL-12p70 has been reported to be suppressed in response to Type I infections (Robben, et al. (2004) supra) (see also FIG. 5C).

Vaccination with cps1-1 induced more rapid systemic production of IFN-γ and Il-12p40 than observed with RH infection. Significant IL-12p70 was produced with cps1-1 vaccination but not in RH infection. It has been reported that an immune evasion may be occurring in RH infection leading to a loss of control of the virulent Type I parasite with systemic overproduction of inflammatory cytokines and lethal pathology (Gavrilescu & Denkers (2001) J. Immunol. 167:902-909; Aliberti, et al. (2003) Nat. Immunol. 4:485-490). Both IL-12-dependent and -independent IFN-γ production has been shown to be required for the development of long term protective immunity leading to control of the chronic infection (Scharton-Kersten, et al. (1996) Exp. Parasitol. 84:102-114). IL-12 dependent IFN-γ production in particular is thought to be required for the development of long lasting protection (Gazzinelli, et al. (1994) J. Immunol. 153:2533-2543). The results presented herein indicate the lack of production of IL12p70 in response to RH would likely add to the inability of the host to directly control the RH infection. As observed during cps1-1 vaccination, IL-12p70 is produced systemically early and maintained, thereby potentially enhancing the overall immune response and leading to the development of more effective long lasting protective immunity.

The Immune Response of IFN-γ, IL-12p40, and IL-12p70 Production is Primarily a Local Response at the Site of cps1-1 Vaccination. As demonstrated herein, a unique pattern of inflammatory cell infiltration occurs during cps1-1 vaccination compared to RH infection. It was important then to measure the kinetics and magnitude of Th-1 cytokine production locally at the site of inoculation and at a peripheral site to ascertain how the local immune response may contribute to control of infection and the development of long lasting immune protection. C57Bl/6 mice were vaccinated i.p. with cps1-1 or infected i.p. with RH, and PECs and splenocytes were harvested at Day 0, Day 2, Day 4, Day 6, and Day 8 post-infection. Harvested PECs and splenocytes were cultured 24 hours at 1×10⁶ and 5×10⁶ cells/ml, respectively, and supernatants recovered from individual cell cultures were used for ELISA to measure the production of IFN-γ, IL-12p40, and IL-12p70.

PECs from mice infected with RH produced significantly more IFN-γ (p<0.001) at Day 4 post-infection than at Day 0 and Day 2 post-infection and remained at a similar level through Day 6 and Day 8 post-infection (FIG. 6A). IL-12p40 (FIG. 6B) was produced by PECs most significantly (p<0.001) at Day 4 post-infection (8-fold increase) as compared to Day 0 and Day 2. IL-12p40 levels subsequently decreased at Day 6 and fell below detectable levels by Day 8 post-infection. Moderate but highly variable levels of IL-12p70 were produced by PECs from RH-infected mice beginning on Day 4 then waning on Day 6 and Day 8 (FIG. 6C).

In comparison to PECs, splenocytes from RH-infected mice produced IFN-γ at greater levels than PECs (FIG. 6D) (Mordue & Sibley (2003) J. Leukoc. Biol. 74:1015-1025; Gavrilescu & Denkers (2001) J. Immunol. 167:902-909). IFN-γ production by splenocytes was not significantly increased until Day 4 post-infection as compared to Day 2 post-infection with a p=0.001 and IFN-γ levels increased on Day 6 and again increased by Day 8. When comparing IL-12p40 production between PECs and splenocytes in RH infection (compare FIG. 6B and FIG. 6E), the production of IL-12p40 appeared equivalent or higher in PECs when taking into account total cell number. Splenocytes produced IL-12p40 in a similar pattern to IFN-γ, where a significant increase in production (p<0.001) in response to RH infection was observed by Day 4, a further increase by Day 6 post-infection, and subsequent decline (FIG. 6E). Minimal production of IL-12p70 by splenocytes in response to RH infection was observed only on Day 4 and Day 6 post-infection (FIG. 6F).

In the case of cps1-1 vaccination, a novel and primarily local pattern of cytokine production by PEC and splenocyte populations was seen as compared to RH infection (FIG. 6). After cps1-1 vaccination, PEC IFN-γ production was detected by Day 2 and increased to highest levels by Day 6 as compared to Day 2 post-infection (p=0.04). Subsequently, IFN-γ levels significantly decreased by Day 8. Unexpectedly, PECs from cps1-1 vaccinated mice produced significantly greater IFN-γ levels than PECs from RH-infected mice by Day 6 post-infection with p=0.005 (FIG. 6A).

As shown in FIG. 6B, cps1-1 vaccination induced a significant production of IL-12p40 by PECs by Day 2 post-infection as compared to that of Day 0 (p=0.04). Although PEC production of IL-12p40 from Day 2 through Day 8 did not significantly change, PEC from cps1-1-vaccinated mice more rapidly produced significant levels (p=0.025) than were observe in RH-infected mice at Day 2 post-infection (FIG. 6B). The opposite was true by Day 4, where PEC derived from RH-infected mice produced significantly more IL-12p40 (p<0.001) than PECs from cps1-1 vaccinated mice. In FIG. 6C it is revealed that PECs from cps1-1-vaccinated mice rapidly produce significantly greater levels of IL-12p70 by Day 2 post-infection as compared to RH with p=0.007. These IL-12p70 levels remained consistently high (Day 2 to Day 8) after cps1-1 vaccination, while Il-12p70 production from PECs after RH infection was delayed to Day 4 and was significantly lower by Day 6 and Day 8 with p=0.012 and p=0.001, respectively (FIG. 6C).

When comparing PECs cytokine production to that of splenocytes from cps1-1-vaccinated mice, higher PEC production (on a per cell basis) of IFN-γ (FIG. 6A and FIG. 6D) and IL-12p70 (FIG. 6C and FIG. 6F) was observed, while splenocyte production of IL12p40 from cps1-1-vaccinated mice was equivalent to that of PEC-production of this protein (FIG. 6B and FIG. 6E). These studies reveal that local production of Th-1 cytokines was more rapid and greater in response to cps1-1 vaccination. The data presented herein also indicates that the initial loss of control of the virulent RH infection may be due to a lack of an early potent cytokine response at the local site of infection. Significantly, it was observed that the immune response directed to a live and invasive parasite in the absence of replication-associated host cell and tissue damage is rapid and tightly controlled (FIGS. 4-6). These data indicate that transient and early (Day 2) systemic IFN-γ production and local IFN-γ production (Day 2 to Day 8), along with early and maintained (Day 2 to Day 8) IL-12p70 production both locally and systemically are sufficient to induce the development of long lasting protective immunity by the cps1-1 vaccine which is very effective at protecting against lethal challenge.

Model of a Local and Tightly Regulated Th-1 Immune Response to Vaccination With a Live, Non-Replicating Parasite in the Absence of Infection-Associated Host Tissue Destruction. Based on the results of analysis conducted herein, the following integrated kinetic model of the immune response to the attenuated non-replicating Type I parasite cps1-1 vaccine is contemplated. Inoculation i.p. with cps1-1 results in a rapid early recruitment of GR-1⁺ CD68⁺ granulocytes and GR-1⁺ CD68⁺ inflammatory macrophages into the local site of inoculation by Day 2. By Day 2 post-inoculation, some percentage of PEC-derived T. gondii-specific granulocytes and/or inflammatory macrophages has migrated peripherally to the spleen based on splenocyte production of IL-12p40 and IL-12p70 by Day 2 post-inoculation. It is possible that a small number of T. gondii-specific CD4⁺ and/or CD8⁺ T cells have also migrated to the spleen by Day 2. While the percentage of CD4⁺ T cells does not rise until Day 4, by Day 2, the absolute number of CD4⁺ T cells is slightly increased. Unexpectedly, the infiltration of CD8⁺ T cells is more significant compared to CD4⁺ T cells at Day 2. There is a transient systemic production of IFN-γ that is only detected at Day 2 that may be explained by the migration of T. gondii cells activated in the peritoneum to the spleen, or other lymphatic tissue. This indicates that by Day 2, the cps1-1 vaccine has already triggered immune surveillance to identify all locations and tissue where T. gondii parasites may have disseminated. However, because the vast majority of cps1-1-invaded cells remain at the original site of inoculation i.p., the Th-1-biased immune response amplifies locally in a tightly controlled manner, and at most, very minor peripheral or systemic responses develop. It appears that the cps1-1-vaccinated host has already committed to a Th-1-biased immune response by Day 2 based on cell infiltration and cytokine production profiles. In part, this early expansion via infiltration of inflammatory and adaptive Th-1 cell types is inversely proportional to a migration of B cell populations out of the peritoneum by Day 2. The retention of B cells is stable though reduced. The tightly controlled Th-1 response may explain the eventual production of T. gondii-specific IgG1 and IgG2a antibody subclasses. It is inferred that the Th-1 response is tightly controlled based on the stable production of systemic- and PEC-derived IL-12p70 from Day 2 to Day 8, along with the low levels of IFN-γ observed at Day 2 that is likely to be derived from the infiltrating granulocytes and/or inflammatory macrophages. While IFN-γ is not detectable (<10 pg/ml) systemically after Day 2, the PEC-derived IFN-γ slightly increases by Day 4, peaks at Day 6, then declines markedly at Day 8, indicating that IFN-γ in the peritoneum after Day 2 correlates closely to the same kinetic pattern as the percentage of CD8⁺ T cells present at the local site of vaccination. CD4⁺ T cells may contribute to PEC-derived IFN-γ although the continued rise in CD4⁺ T cells at Day 8 does not correlate with the significant decline in IFN-γ production between Day 6 and Day 8. The innate response subsides between Day 2 and Day 4 based on loss of systemic IFN-γ and the marked loss of granulocytes and inflammatory macrophages that infiltrate between Day 0 and Day 2. The slight increase in inflammatory macrophages between Day 4 and Day 6 indicates that some percentage of T gondii-specific Gr-1⁺ CD68⁺ cells that left the peritoneum on Day 2 to search peripheral organs has returned to the peritoneum where most of the originally vaccinated parasites remain locally positioned. These returning Gr-1⁺ CD68⁺ inflammatory macrophages or infiltrating T-reg cells may suppress the Th-1 response and explain the decline in IFN-γ between Day 6 and Day 8. The Day 4 to Day 6 Gr-1⁺ CD68⁺ increase cannot by itself explain the marked increase in IFN-γ between Day 4 and Day 6 and these inflammatory macrophages largely depart the peritoneum by Day 8. The cross-talk between CD4⁺ and CD8⁺ T cells may explain the peak of IFN-γ production on Day 6. The decline of IFN-γ by Day 8 indicates the local Th-1 inflammatory response is rapidly resolving. In regard to T cells, the data herein cannot discriminate between two models where the marked increase in CD4⁺ and CD8⁺ T cells at Day 6 (stable to day) is from continued infiltration of new T cells to the peritoneum or alternatively is due to IFN-γ-dependent expansion of previously peritoneum-activated T gondii-specific T cells. It is contemplated that the complete CD4⁺ and CD8⁺ T cell response is determined by cell infiltration, antigen presentation, and signaling events that have occurred by Day 2 post-vaccination with cps1-1. The Gr-1⁺ CD68⁺ inflammatory macrophage population may play a role in antigen presentation to CD4⁺ and CD8⁺ T cells. Infected epithelial cells or other cps1-1-invaded cell types are also likely to present antigen to T cells. The rapid production of IL-12p70 by Day 2 and its production maintained through Day 8 by PECs is likely important for tight regulation of the Th-1 immune response. The source of PEC-derived IL-12p70 is under investigation. Early production of IL-12p70 by Day 2 may originate from the Gr-1⁺ CD68⁺ granulocytes and Gr-1⁺ CD68⁺ inflammatory macrophages (Bennouna, et al. (2003) J. Immunol. 171: 6052-6058; Bliss, et al. (2000) J. Immunol. 165:4515-4521). However, IL-12p70 production rises significantly by Day 4 while the granulocyte population essentially disappears, and based on cell type profiles in the peritoneum the only profile that correlates precisely to production of IL12p70 is the CD19⁺ B cell. The immune response elicited to cps1-1 vaccination is local and rapid, and the inflammatory response is tightly regulated. This immune response leads to a very effective and long lasting immunity to T. gondii.

Example 6 T. gondii as a Delivery Vector for Heterologous Antigens

By way of illustration, gene specific primers are generated to amplify the coding sequence for P. berghei merozoite surface protein-1 (MSP-1), the sequence of which is known in the art under GENBANK Accession No. XP_(—)678505. The amplified product fused to the SAG1 promoter (Striepen, et al. (1998) supra) and cloned into p53KOX3-1R, i.e., the CPSII deletion construct which harbors the DHFR-TK-TS marker sequences. The resulting construct is introduced into T. gondii using established methods and an attenuated uracil auxotroph which expresses MSP-1 is identified based on dependence upon pyrimidine supplementation for replication and expression of MSP-1. A suitable murine Plasmodium model is used to demonstrate protective immune responses of the T. gondii-based vaccine to P. berghei infection. Immune response to Plasmodium parasites is associated with reduction in patient infection intensity. With this invention, the potency of immune response against MSP-1 and other malarial antigens is expected.

Previous work has demonstrated the feasibility of using live vectors for immunization against malaria. Immunization of mice with Salmonella expressing CSP and MSP-1 protected against P. berghei, and induced immune responses against P. falciparum MSP-1 (Sadoff (1988) Science 240:336-8; Toebe (1997) Am. J. Trop. Med. Hyg. 56:192-9; Wu (2000) Biotechnol. 83:125-35). The anti-MSP-1 immune response did not require secretion of antigen from the bacterium or surface display. These data indicate that use of T. gondii as a platform to deliver P. berghei antigens in vivo is highly likely to protect mice and other mammals against malaria. For use in humans, vaccines that work in the P. berghei mouse model can be reconstructed with homologous P. falciparum antigens. Safety and immunogenicity testing in mice and efficacy testing against infection in nonhuman primates can then lead to human trials.

Other antigens and animal models are well-known in the art and can be employed in accordance with the present invention. For example, the B. anthracis protective antigen can be expressed by the T. gondii cps1-1-based vector platform with protection against anthrax infection determined using either the well-established mouse or guinea pig model (Peterson et al. (2006) Infect. Immun. 74:1016-24).

Example 7 Essential Indels and Domains of CPSII

Plasmid Construction. A functional CPSII minigene encoding the authentic 1687 amino acids of carbamoyl phosphate synthetase was constructed by sequential coupling of defined cDNA segments generated by reverse transcriptase/PCR. First, a 1829 bp cDNA for the N-terminal GATase domain of CPSII was amplified from polyA+mRNA from the RH strain (5′-ACT AGT GGT GAT GAC GAC GAC AAG ATG CCT CAC AGT GGA GGG C-3′, SEQ ID NO:12; and 5′-GAT ATC CAC GTG TCG CGG CCG CGC TCT C-3′, SEQ ID NO:13). The 1829 bp cDNA was introduced (SpeI/EcoRV) into PET41b (SpeI/XhoI-blunted). Next an N-terminal section of the CPS domain cDNA including by 1829 to by 3532 was generated (5′-GAG AGC GCG GCC GCG AC-3′, SEQ ID NO:14; and 5′-CAC GTG GAG GCG AGA CGT CGT CGT C-3′, SEQ ID NO:15) and fused to the GATase domain (NotI/PmlI). The remainder of the CPS domain was constructed by amplifying two cDNA segments, by 3003-4097 (5′-AGT ACT TGA TGA ATT CAC CG-3′, SEQ ID NO:16; and 5′-TTT CTG CGA GAT CTT CTT CAC G-3′, SEQ ID NO:17) and by 4097-5065 (5′-GCG TGA AGA AGA TCT CGC AG-3′, SEQ ID NO:18; and 5′-ATC GAT CAC GTG ATT TTT GAG GCC AGT ATT CAT CC-3′, SEQ ID NO:19), and then the two C-terminal segments were fused in PCR4TOPO (EcoRI/BglII). Finally the C-terminal section of CPS was fused with the N-terminal section in PET41b (EcoRI/PmlI) and the complete 5063 bp CPSII minigene coding sequence was determined to verify authenticity.

5′ UTR and 3′ UTR were amplified from RH genomic DNA. 5′ UTR to by −516 was amplified (5′-GCT AGC GTG GAC CCC CAT TAT CCT TCG C-3′, SEQ ID NO:20; and 5′-ACT AGT CAC TCG TCG AAT GGT TGC GTC TG-3′, SEQ ID NO:21), and 5′ UTR to by −2057 was amplified (5′-GCT AGC GTG GAC CCC CAT TAT CCT TCG C-3′, SEQ ID NO:22; and 5′-ACT AGT GAA ATC GCG ATC AAC GCG ACA G-3′, SEQ ID NO:23). The 3′ UTR (920 bp) was amplified (5′-AGT ACT TGC ACC ACC ACC ACC ACC ACT AAT TTC CAA TAC TTT CGC CAA AAA CGT TCC-3′, SEQ ID NO:24; and 5′-GCG CAC GTG GTT GAG AGC TTG ACC CGC ATG CA-3′, SEQ ID NO:25). Finally 5′ UTR segments (ScaI/SpeI) were fused into the CPSII minigene (SpeI), and then the 3′ UTR (ScaI/PmlI) was fused to the above plasmid(s) (ScaI/PmlI).

Site-Directed Mutagenesis. Mutations were first introduced into the either the GATase or CPS domains using Stratagene's PCR based QUIKCHANGE® II XL Site-Directed Mutagenesis Kit. Products were DpnI digested, transformed into XL-10 GOLD® Ultracomp cells, and subsequently transferred into the full CPSII complementation vector. Forward and reverse complimentary primers containing the desired mutations were used to create the desired mutations and CPSII minigene mutations were verified prior to transfection experiments.

Parasite Culture and Transfection. Tachyzoites were maintained in human foreskin fibroblasts with or without uracil supplementation (300 mM). Wild-type or CPSII minigene plasmids containing defined mutations were transfected (20 mg) into the cps1-1 background and selections were performed without drug addition in the absence of uracil using previously described methods.

Parasite Growth Assays. The growth of tachyzoites in culture fibroblasts was measured at 36 hours post-transfection, as well as in standard 7 day pfu assays. To determine parasite growth rate (doubling time) tachyzoites per vacuole were scored from 50 randomly selected vacuoles containing 2 or more parasites 36 hours post-transfection. Transient complementation efficiency was measured at 36 hours by counting the number of vacuoles containing 2 or more parasites in 50 randomly selected areas of the culture. The wild-type CPSII minigene was used in control experiments in all transfections to compare efficiency of transient complementation. Stable complementation efficiency was determined in a standard 7 day plaque forming unit assays. Immediately following transfection duplicate cultures were inoculated with 2%, 0.5%, or 0.1% of transfected parasites and pfu were scored 7 days later.

Complementation of Uracil Auxotrophy with Functional CPSII cDNA. The cps1-1 mutant of T. gondii invades host cells, but due to pyrimidine starvation, exhibits no detectable growth rate in the absence of uracil supplementation. Consequently, providing a functional CPSII gene to the cps1-1 mutant can restore production of carbamoyl phosphate required for biosynthesis of UMP. To examine complementation, a cDNA minigene encoding the 1687 amino acid CPSII polypeptide was constructed under the control of authentic CPSII 5′ UTR and 3′ UTR regulatory regions. Plasmids representing a promoter-less minigene coding region construct as well as minigenes under the control of either 0.5 kb or 2.0 kb of 5′ UTR were transfected into the cps1-1 uracil auxotrophic mutant and parasites were cultured in HFF cells in the absence or presence of uracil. Thirty-six hours after transfection, the number of tachyzoites per parasite vacuole was scored by counting 100 randomly chosen vacuoles. The 2 kb 5′ UTR CPSII minigene Pc4 efficiently complemented the cps1-1 mutant and restored a normal tachyzoite growth rate in the absence of uracil. In contrast, both the promoter-less construct Pc0 as well as the 0.5 kb 5′ UTR construct Pc2 failed to complement the cps1-1 mutant and did not restore any detectable growth rate in the absence of uracil. These results demonstrate functional complementation of uracil auxotrophy in T. gondii. By counting the number of vacuoles containing actively replicating tachyzoites in the absence versus the presence of uracil, the overall efficiency of complementation following transfection of the 2 kb 5′ CPSII cDNA minigene was determined. In a 36 hour growth assay, uracil auxotrophy was complemented in ˜34% of parasites surviving electroporation with plasmid Pc4, demonstrating a very high efficiency of complementation roughly equivalent to the efficiency of positive selection reported using plasmids bearing pyrimethamine resistant alleles of dihydrofolate reductase-thymidylate synthase as a selectable marker (Donald & Roos (1993) Proc. Natl. Acad. Sci. USA 90:11703-7). Pc4 transfectants showed a high frequency of stably complemented cps1-1 parasites that exhibited the high virulence phenotype of the parental RH strain in c57/b16 mice.

Functional Analysis of the Glutamine Amidotransferase Domain of CPSII. The requirement of the fused eukaryotic GATase domain to produce ammonia for CPS function in apicomplexan CPSII has not been previously examined in vivo. A mutation in plasmid Pc4 was constructed, wherein an essential catalytic triad residue of the GATase domain (Table 2), equivalent to Cys269 in E. coli Carbamoyl Phosphate Synthetase (Rubino, et al. (1987) J. Biol. Chem. 262:4382-6), was mutated to abolish activity. The resulting Cys345 to A1a345 mutation completely abolished complementation activity, indicating that T. gondii CPSII is dependent on a functional GATase domain for the production of ammonia in vivo (Table 3). The dependence of T. gondii CPSII activity on the amidotransferase domain validates the analysis of unique sites within this domain as parasite specific drug targets.

TABLE 2 Location in T. gondii Mutation CPS II Effect on CPS or CPSII E. c. C269A C345 Mutation in catalytic triad Abolishes GATase activity E. c. G359F G435 Mutation in tunnel wall Ammonia leaks Uncouples GATase & CPS H. a. T456A T533 Mutation in MAPK site Abolishes MAPK activation E. c. E761A E1316 Mutation in K-loop Abolishes ornithine activation Abolishes UMP repression E. c. H781K H1336 Mutation in K-loop Reduces CPS activity Reduces ornithine activation Reduces UMP repression E. c. T974A T1530 Mutation in regulatory-D Reduces ornithine activation Abolishes IMP activation Abolishes UMP repression H. a. S1345A T1530 Mutation in regulatory-D Reduces PRPP activation E. c. T1042A T1649 Mutation in regulatory-D Reduces ornithine binding

TABLE 3 T. gondii Transient Stable CPSII GATase Growth Rate Efficiency Efficiency Mutation (Hours) (% of Wild-Type) (% of Wild-Type) Wild-Type 7.4 100 100 Δ172-229 Nd 0 0 C345A Nd 0 0 N348R Nd 0 0 N348A 8.2 96 91 P385R 11.5 18 1.3 G435F 12.4 8.4 0.6 Δ455-457 7.5 98 103 Δ454-470 8.9 64 11

The proximal Asn348 residue, which is selectively present in most protozoan CPSII enzymes (FIG. 7), was subsequently targeted. Mutation of Asn348 to Arg348 abolished complementation activity, whereas mutation of Asn348 to A1a348 only moderately reduced the initial growth rate (from 7.4 to 8.4 hours) in the 36-hour growth assay, but did not significantly interfere with the efficiency of transient or stable complementation (Table 3). Amino acid 385 adjacent to residues encompassing the catalytic triad is uniquely a proline residue in T. gondii CPSII (FIG. 7). Changing amino acid P385 to R385 had a dramatic effect on reducing the initial parasite growth rate (from 7.4 to 11.5 hours, and reduced transient complementation efficiency to 18% and stable complementation efficiency to 1.3% of the control (Table 3). A reduced efficiency of transient complementation within the primary vacuole indicates that several copies of mutant R385 plasmid per parasite were required to restore growth. This was borne out in real-time PCR analysis of cloned progeny from the pfu assay indicating that between 3 and 7 plasmid copies were stably integrated in complemented parasites.

The requirements for ammonia production from GATase and channeling of ammonia are well-described for E. coli CPS (Huang & Raushel (2000) J. Biol. Chem. 275:26233-40; Huang & Raushel (2000) Biochemistry 39:3240-7; Miles, et al. (1998) Biochemistry 37:16773-9; Thoden, et al. (2002) J. Biol. Chem. 277:39722-7; Thoden, et al. (1999) Acta Crystallogr, D, 8-24). Perforation of the ammonia tunnel in E. coli CPS via mutation of G359 to F359 results in ammonia leakage from the tunnel and loss of CPS activity (Table 2). The Gly residue corresponding to E. coli G359 is universally conserved in all GATases that are coupled with CPS activity (FIG. 7). Mutation of T. gondii G435 to F435, corresponding to the E. coli G359 to F359 mutation, caused a marked reduction in the initial parasite growth rate (from 7.4 to 12.4 hours), and reduced transient complementation efficiency to 8.4% and stable complementation efficiency to 0.6% of the control. These results indicate that disruption of the putative ammonia tunnel markedly decreased CPSII activity in vivo (Table 3), again showing a strict dependence of the parasite CPS on ammonia produced by the fused GATase activity of CPSII.

Deletion of Indels. Apicomplexan CPSII enzymes contain locations where novel insertions of amino acids (indels) occur at several locations within the GATase and CPS domains (FIG. 7). While ribozyme targeting of a P. falciparum CPSII indel at the RNA level was previously shown to inhibit parasite proliferation (Flores, et al. (1997) J. Biol. Chem. 272:16940-5), few studies have directly addressed the functional importance of indels in parasite proteins. The unusually frequent occurrence of novel insertions of low or high complexity within protozoan parasite proteins, particularly in Plasmodium sp. and T. gondii (Cherkasov, et al. (2006) Proteins 62:371-80; DePristo, et al. (2006) Gene 378:19-30), may provide parasite selective drug targets in certain instances where the indel provides a necessary function for biological activity of an essential parasite protein. Functional complementation of CPSII in T. gondii enabled a genetic test of essential indels. In the GATase domain the T. gondii CPSII indel location was targeted where other apicomplexan CPSII also exhibit a large amino acid insertion that other protozoans, fungi, mammals, and prokaryotes do not share (FIG. 7). Deletion of the GATase indel (E171-A229), relative to human GATase, completely abolished CPSII function as demonstrated by the inability of this mutant to complement the uracil auxotrophy of cps1-1 (Table 3), and established this indel as a parasite-selective drug target within the essential GATase domain.

The carboxy terminal region of the CPSII.A domain (domain A3) contains the oligomerization domain known to coordinate the formation of tetramers of E. coli CPS (Kim & Raushel (2001) Biochemistry 40:11030-6; Thoden, et al. (1997) Biochemistry 36:6305-16). On the N-terminal side of the putative CPSII oligomerization domain a novel indel of ˜34 amino acids is present in T. gondii CPSII (FIG. 7). Deletion of this indel (C873-G910) caused a minor, but detectable, disruption in complementation activity based on a reduced initial growth rate (from 7.4 to 8.2 hours), similar transient complementation efficiency (113%), and slightly reduced stable complementation efficiency to 65% of the control (Table 4). The more subtle effect of this indel deletion in the T. gondii CPSII oligomerization domain is potentially similar to the minor effect on E. coli CPS activity previously observed in mutants blocked in oligomerization contact regions that prevent tetramer but not dimer formation (Kim & Raushel (2001) supra).

TABLE 4 Transient Stable T. gondii Growth Rate Efficiency Efficiency CPS Mutation (Hours) (% of Wild-Type) (% of Wild-Type) Wild-Type 7.4 100 100 T533A 7.4 105 98 S581A 7.3 109 104 Δ873-910 8.2 113 65 E1316A Nd 0 0 E1318A Nd 0 0 H1336K Nd 0 0 T1430A 7.7 86 82 T1530A 8.6 51 10 T1530 fs Nd 0 0 Δ1592-1628 Nd 0 0 S1608A Nd 0 0 T1649A 8.5 65 37

Interaction of allosteric effectors with the C-terminal regulatory domain directly trigger conformational changes in CPS affecting activity and/or synchronization of active sites (Thoden, et al. (1999) Acta Crystallogr. D Biol. Crystallogr. 55:8-24). T. gondii and B. bovis CPSII share an indel location within the C-terminal regulatory domain (FIG. 7). To examine whether this novel indel was essential to CPSII function, a deletion of the C-terminal indel (G1592 to R1628) was constructed. This deletion completely abolished CPSII complementation activity (Table 4). Remarkably, only a point mutation at residue S1608 to A1608 in this indel was necessary to abrogate the ability of the CPSII minigene to complement the uracil auxotrophy of cps1-1. These results indicate that the C-terminal regulatory region indel represents a parasite-selective drug target.

CPSII Regulatory Domains. Suppression of mammalian CPSII activity is highly dependent on the presence or absence of regulated phosphorylation at a distinct MAP kinase site at (T456) in the carboxy phosphate CPSII.A domain by MAPK (Graves, et al. (2000) Nature 403:328-32). T. gondii and other lower eukaryotic forms of CPSII are distinct from mammalian CPSII in lacking this critical MAPK site (FIG. 7). However, since T. gondii CPSII shares the Threonine residue corresponding to the mammalian T456 position, this residue was mutated to exclude the possibility that a novel parasite MAP, or a novel, kinase may control CPSII activation. Mutation of T533 to A533 in T. gondii CPSII had no significant effect on complementation activity (Table 4). It was also determined whether the nearby putative MAPK core SP site present in T. gondii but absent in mammalian CPSII was necessary for activity. Mutation of 5581 to A581 also had no detectable effect on complementation activity (Table 4).

CPS is controlled via allosteric mechanisms acting through specific allosteric effectors and their binding interactions with the C-terminal domain of CPS.B (Braxton, et al. (2000) Biochemistry 38:1394-401; Fresquet, et al. (1999) J. Mol. Biol. 299:979-91; Pierrat, et al. (2002) Arch. Biochem. Biophys. 400:26-33; Thoden, et al. (1999) J. Biol. Chem. 274:22502-7). Prokaryotic (E. coli) CPS activity is repressed by UMP, strongly activated by ornithine and weakly activated by IMP, whereas eukaryotic CPSII is typically activated by PRPP and is repressed by UTP (or UDP in kinetiplastids) (Jones (1980) Ann. Rev. Biochem. 49:253-79; Nara, et al. (1998) Biochim. Biophys. Acta 1387:462-8). Strikingly, T. gondii CPSII is insensitive to allosteric activation by PRPP, and relatively high levels of UTP are required for suppression (Asai, et al. (1983) Mol. Biochem. Parasitol. 7:89-100). To gain further insight into the importance of allosteric regulatory regions and the type of regulation occurring in T. gondii CPSII, mutations were constructed in several amino acid residues that were conserved between the T. gondii and E. coli C-terminal regulatory domains, and that were also known to mediate allosteric control in E. coli CPS (Table 2). While IMP induces only modest allosteric effects on E. coli CPS activity, ornithine potently activates the glutamine dependent ATPase and ATP synthesis reactions and thereby markedly upregulates activity by increasing the affinity of CPS for its nucleotide substrates while overriding the strong effect of UMP to suppress the catalytic activity of CPS (Braxton, et al. (1999) supra). Targeting of the conserved ornithine binding sites identified in T. gondii CPSII analogous to those previously identified in E. coli (Table 2) was carried out. The K loop coordinates the binding of a potassium ion and includes the conserved residue H781 that also coordinates the transmission of the allosteric regulatory signals from the C-terminal regulatory domain (Pierrat, et al. (2002) supra; Thoden, et al. (1999) supra). Mutation of H781 to K781 in E. coli CPS reduces the magnitude of the allosteric effects of both ornithine and UMP, decreases the allosteric response to IMP, and also diminishes the catalytic activity of CPS by one to two orders of magnitude in the absence of allosteric effectors (Pierrat, et al. (2002) supra). In T. gondii, it was found that a CPSII minigene with the analogous mutation (H1336 to K1336) failed to detectably complement the uracil auxotrophy of the cps1-1 mutant (Table 4). A second mutation (E761 to A761) also within the K loop of E. coli CPS was previously found to be crucial to the transmission of the allosteric activation signal by ornithine, but did not affect catalytic turnover in the absence of effectors. This mutation also eliminated feedback repression by UMP and decreased activation by IMP (Pierrat, et al. (2002) supra). In T. gondii CPSII, it was observed that the analogous mutation of E1316 to A1316 resulted in a complete loss of complementation activity by the mutant CPSII minigene (Table 4). Interestingly, a second mutation (E1318A to A1318) at a nonconserved residue two amino acids downstream of E1316 also resulted in a complete loss of complementation activity by the mutant CPSII minigene (Table 4).

To further define the extent of the impact of mutations within the allosteric regulatory region, a residue conserved between T. gondii and the E. coli CPS.B3 allosteric regulatory region (T974) that strongly influenced the allosteric response to ornithine in E. coli CPS (Table 2) was also targeted. The mutation T974 to A974 disrupts the IMP/UMP binding pocket in E. coli CPS and not only abolishes UMP inhibition and IMP activation, but also decreases activation by ornithine (Fresquet, et al. (2000) supra). Interestingly, this site also plays a role in allosteric control in mammalian CPSII as well since mutation of the corresponding hamster residue 51355 to A1355 nearly abolishes activation by PRPP and lowers overall CPSII activity ˜5-fold (Simmons, et al. (2004) Biochem. J. 378:991-8). While T. gondii CPSII is nonresponsive to PRPP in vitro, mutation of the analogous residue in T. gondii CPSII (T1530 to A1530) significantly reduced CPSII complementation activity based on a reduced initial parasite growth rate (from 7.4 to 8.6 hours), and reduced transient complementation efficiency to 51% and stable complementation efficiency to 10% of the control suggesting several copies of this mutant allele are necessary to fully support parasite growth (Table 4). A second conserved residue (T1042) in the E. coli regulatory domain plays a direct role in ornithine binding (Pierrat, et al. (2002) supra). Mutation of T1042 to A1042 in E. coli CPS reduces the magnitude of the allosteric response to ornithine. The analogous mutation T1649 to A1649 in T. gondii had the more modest effect of slightly lowering the initial growth rate (from 7.4 to 8.5 hours), and reduced transient complementation efficiency to 65% and stable complementation efficiency to 37% of the control. Additionally, the overall importance of the relatively nonconserved T. gondii CPSII C-terminal regulatory region as a putative drug target could readily be seen through a frame-shift that was incorporated into T1530 that effectively deleted much of the regulatory region (Table 4). This mutation abolished complementation of cps1-1 by the CPSII minigene. 

What is claimed is:
 1. An attenuated uracil auxotroph mutant of Toxoplasma gondii comprising a nucleic acid molecule encoding an exogenous protein, wherein the exogenous protein is a non-Toxoplasma gondii bacterial, fungal, parasitic or tumor antigen, a protein that produces a non-Toxoplasma gondii antigen, a therapeutic antibody, an interferon, blood factor, insulin, erythropoietin, blood clotting factor, asparaginase, catalase, lipase, or tissue plasminogen activator.
 2. The mutant of claim 1, wherein said mutant lacks a functional carbamoyl phosphate synthetase II enzyme.
 3. A vaccine for protection against infection by T. gondii and a non-T. gondii disease comprising the mutant of claim 1 in admixture with a pharmaceutically acceptable carrier.
 4. The vaccine of claim 3, wherein said vaccine is formulated for peritoneal, oral, topical, or intravenous administration.
 5. A method for generating an immune response in an animal comprising administering to an animal an effective amount of the mutant of claim 1 so that an immune response to Toxoplasma gondii or non-T. gondii antigen is generated.
 6. A method for identifying an effector of a human therapeutic target protein or enzyme comprising contacting the mutant of claim with an effector and determining whether the effector inhibits or activates the human therapeutic target protein or enzyme.
 7. A method for sensing a host cell environment or specific molecules present or absent in the host cell environment comprising administering the mutant of claim 1 to a subject and determining the host cell environment or presence or absence of specific molecules in the host cell environment.
 8. The method of claim 7, wherein the molecule is a pyrimidine. 